Textbooks are among the best ways to familiarize yourself with a subject. However, they can sometimes be expensive. Fortunately, there are various freely available textbooks online. They may not cover everything that textbooks that cost money do and they may not be as reliable, detailed or as prestigious, but they can serve as a viable introduction for curious individuals. Therefore, I cannot guarantee that every book in this list is 100% accurate, but I try to pick reputable textbooks. I have not included very old textbooks (although be careful with trusting older books too much), or certain open access textbooks that promote pseudoscience like alternative medicine in some of its chapters. I have also not included textbooks that have been uploaded illegally. In other words, all of the textbooks in this thread can be read or obtained legally. Finally, some of these “textbooks” are actually comprehensive lecture notes, so please count them as functional textbooks.
Note: There are probably hundreds more of this stuff out there on the Internet. However, there comes a time when I might as well publish the list of the ones that I have so far, as this will always be a work in progress. Enjoy.
Note: See a textbook or set of lecture notes that is missing from this list? Post your discovery in the comment section and I’ll add it to the list, without attribution, if it makes sense.
Biology
Free Introductory Biology Textbooks
- Rediscovering Biology: Molecular to Global Perspectives – a textbook that covers advances in biology during the last few decades. Covers genomics, proteins and proteomics, evolution and phylogenies, microbial diversity, emerging infectious diseases, HIV/AIDS, developmental genetics, cell biology and cancer, human evolution, neurobiology, biodiversity, biology of sex and gender and genetically modified organisms.
- Inside the Cell – very introductory textbook in cell biology, covering areas such as organelles, cellular reproduction and cancer.
- On-Line Biology Book – a introductory biological textbook looking at chemistry of cells, cell division, human genetics, plant structure, nervous system, digestive system, evolution and paleobiology and biological diversity.
Free Molecular Biology, Genetics and Cancer Textbooks
- The New Genetics – shorter primer on recent advances in genetics. Covers microarrays, RNA inference, imprinting and epigenetics, recombinant DNA technology, PCR, genome evolution, genome sequence, forensics,
- Genetics for Surgeons – a basic textbook in genetics, focusing on general principles of genetics, genetics disorders that a surgeon can come across and familial cancer.
- Genomes (2nd edition) – another textbook in genetics, looking at general principles, new advances in genomics, genome sequencing, assembly of transcription initiation, genome regulation and evolution as well as molecular phylogeny.
- An Introduction to Genetic Analysis (7th edition) – this textbook focuses on genetic mapping, but has large coverage of gene function and transcription as well as DNA replication and recombinant DNA technology. Also has several chapters devoted to mutations, development, cancer, population, quantitative and evolutionary genetics.
- Weed To Wonder – a short online textbook on the scientific background to artificial selection of corn. Covers topics such as domestication, hybrid vigor, genome sequencing, mobile genetic elements and biofortification.
- Molecular Biology of the Cell (4th edition) – a searchable version of the 4th edition of the popular textbook Molecular Biology of the Cell by Bruce Alberts and others. It cannot be browsed directly, so search the different headlines in the table of contents to read the book from start to finish. Requires cunning uses of the search feature.
- Molecular Cell Biology (4th edition) – another classic molecular biology textbook by Lodish and others available at NCBI.
- The Cell: A Molecular Approach – yet another general molecular biology textbook by Cooper at NCBI.
- Dynamics of Cancer: Incidence, Inheritance, and Evolution – textbook examining cancer on the level of epidemiology, molecular biology, progression and evolution.
- Holland-Frei Cancer Medicine – another textbook about cancer, includes topics such as cancer and apoptosis, growth factors and signal transduction, oncogenes, tumor suppressor genes, invasion and metastasis as well as cancer immunology.
Free Evolution Textbooks
- 29+ Evidences for Macroevolution: The Scientific Case for Common Descent – a textbook on the evidence for evolutionary common descent.
- Science, Evolution, and Creationism – a textbook on the evidence for evolution and the problems with creationism.
- In the Light of Evolution: Volume I. Adaptation and Complex Design – textbook looking at multicellularity, the emergence of biocomplexity, symbiosis, adaptive evolution of color vision and the problems with creationism.
- In the Light of Evolution, Volume II: Biodiversity and Extinction – the second volume covers ecological extinction, species invasion and extinction and the future of mammalian biodiversity.
- In the Light of Evolution III: Two Centuries of Darwin – the third volume looks at natural selection during speciation, adaptive radiations, mate choice and sexual selection, reproductive decisions under ecological constraints as well as some interesting philosophical articles by Dennett, Ruse and Ayala.
- In the Light of Evolution IV: The Human Condition – the fourth volume details human evolution, human skin pigmentation, coevolution of intelligence, sociality, and language and finally morality.
- In the Light of Evolution V: Cooperation and Conflict – the final volume in this series looks at kin selection, social evolution, evolution of drug resistance and genomic imprinting with regards to evolutionary psychology.
Free Immunology, Vaccine and Epidemiology Textbooks
- Immunology – shorter textbook on immunology covering complement, structure of antibodies, tumor immunology, regulation of immune system, immunization, autoimmunity, hypersensitivities and immunodeficiencies.
- Immunology and Evolution of Infectious Disease – introductory textbook in immunology that covers the basics. Covers vertebrate immunology, benefits and molecular background of antigenic variation, immunological memory, and the evolution of the immune system.
- Immunobiology – the 5th edition of his classic textbook on immunology. It cannot be browsed directly, so search the different headlines in the table of contents to read the book from start to finish. Requires cunning uses of the search feature.
- Epidemiology and Prevention of Vaccine-Preventable Diseases – a comprehensive textbook on vaccines, covering principles of vaccination, vaccine safety, and covers the majority of vaccine-preventable diseases and their associated vaccines, such as measles, influenza, polio and HPV.
- Epidemiology for the Uninitiated – a short textbook on the basics of epidemiology provided by British Medical Journal
- Basic Epidemiology – an epidemiology textbook from WHO. Good set up with key messages and easy-to-understand mathematics.
- Human Herpesviruses: Biology, Therapy, and Immunoprophylaxis – an encyclopedia-like textbook on the human hepresviruses.
Free Pharmacology, Endocrinology, Regenerative Medicine and Stem Cells Textbooks
- Medicines by Design – a book on drug development, covering basic pharmacology, medicines isolated from nature, and how scientists go from molecules to medicines.
- The Structures of Life – A shorter primer on proteins, x-ray crystallography, nuclear magnetic resonance, structure-based drug design.
- Endocrinology: An Integrated Approach – a solid textbook on endocrinology looking at general principles of hormones, the structure and function of different glands such as gonads, pituitary, adrenal and thyroid glads. and their associated diseases.
- Regenerative Medicine – report on stem cells by the Department of Health and Human Services covering current progress and future perspectives.
- Stem Cells: Scientific Progress and Future Research Directions – a larger textbook on stem cells. Topics covered include embryonic and adult stem cells, stem cells and diabetes, gene therapies etc.
- StemBook – peer-reviewed chapters by experts in stem cell biology. More advanced than the other books on stem cells in this list as this one details topics such as regulation and function of small RNAs in embryonic stem cells, the relationship between stem cells, cancer and epigenetics, the neural stem cell microenvironment, immunologic targeting of the cancer stem cell, tissue engineering and molecular imaging of stem cells.
Free Neurology and Neurobiology Textbooks
- Physical Neuroscience Lecture Notes – almost 200+ pages of lecture notes on physical neuroscience. Covers action potentials, ion channels, neurotransmitters, ligand-gated receptors, intracellular signaling, dendritic integration and computational neuroscience.
- Neuroscience – an earlier edition of a classic text in neuroscience by Purves and others. Covers organization, signaling and the effects of neurotransmitters, sensation and sensory processes, development and complex brain functions, memory, sex and emotion.
- Neurobiology of Sensation and Reward – looks at the history of reward research, detailed overview of different senses, and the neurological connection between sensations and rewards as well as human rewards such as perfume, visual arts, and music.
- Basic Neurochemistry – covers inter- and intracellular signaling, growth, development and differentiation, inherited and neurodegenerative diseases as well as neural processing and behavior such as the molecular biology of vision and neurochemistry of Schizophrenia.
- The Neurobiology of Olfaction – an anthology on recent advances in the field. Topics include olfaction in C. elegans, insects and Xenopus, the development of the olfactory system, pheromones and mammalian behavior and of course the basics steps in human olfactory processing.
Miscellaneous Free Biology Textbooks
- Testing Treatments: Better Research for Better Healthcare (2nd edition) – A short textbook on the value and application of evidence-based medicine. How do we know a treatment works?
- The Merck Manual of Diagnosis and Therapy – a diagnostic handbook detailing various diseases and disorders.
- WormBook – the best book on the nematode C. elegans.
- Essentials of Glycobiology – a book from Cold Spring Harbor Laboratory Press (2009) with lots of details on stuff like evolution of glycan diversity, discovery and classification of glycan-binding proteins and glycans in physiology and disease (e. g. parasitic infections).
- Developmental Biology – an earlier edition of the classic textbook by Gilbert. Topics include genes and development, ethical issues, cell-cell communication in development, early development in invertebrates, insects, birds, fish and mammals, development of tetrapod limb, sex differentiation and plant development as well as developmental mechanisms of evolutionary change.
- Grey’s Anatomy of the Human Body – a classic text in anatomy.
- Development of the Human Heart – a fairy short introduction to the development of human heart.
- Online Textbook of Bacteriology – a basic overview of bacterial microbiology that looks at topics such as structure and function of procaryotes, mechanisms of pathogenicity, interactions with the immune system as well as case studies looking at specific microorganisms that cause disease in humans.
- Bacteriology – another bacteriology textbook looking at both basic stuff like structure, bacteriophages, horizontal gene transfer and more complex topics like gene regulation and specific disease-causing bacteria.
- Virology – textbook about viruses. Talks about stuff like replication strategies, viral genetics, HIV, chemotherapy against viruses, virus-host interactions and various specific types of viruses.
- Parasitology – short textbook about parasites such as ticks, nematodes, malaria and intestinal parasites.
- Mycology – describes Actinomycetes, yeasts and opportunistic mycoses.
Chemistry
Free Introductory and General Chemistry Textbooks
- General Chemistry I Course Notes – almost 200 pages course notes in general chemistry. Details stuff like moles, the periodic table, chemical equations and stoichiometry etc.
Free Biochemistry Textbooks
- Biochemistry – an older edition of the classic text by Berg, Tymoczko and Stryer. A general textbook in biochemistry. Covers biochemical evolution, protein structure and function, exploration of genes and evolution, enzymes and kinetics, metabolism and signal transduction.
- Biochemistry Online: An Approach Based on Chemical Logic – an online textbook on biochemistry. Covers lipid and protein structure, carbohydrates, DNA, genomics, binding, transport, kinetics, catalysis, phosphorylation, oxidation, signal transduction and metabolic pathways.
- Biochemistry Lecture Notes I – not precisely a textbook, but a full set of biochemistry lecture notes from Humboldt State University.
- Biochemistry Lecture Notes II – extensive lecture notes on more advanced topics.
- Medical Biochemistry Concepts – another non-textbook textbook, but loads of material spread out onto different pages.
Free Physical Chemistry Textbooks
- Physical Chemistry in Brief – 450+ page textbook in physical chemistry. Covers topics such as basic terms of physical chemistry, state behavior, fundamentals of thermodynamics, thermochemistry, chemical equilibrium and kinetics, electrochemistry as well as the physical chemistry surfaces and dispersion systems.
- Introductory Physical Chemistry I – 100+ lecture notes on physical chemistry, looking at stuff like Maxwell-Boltzmann distribution, thermochemistry, Carnot Engine, entropy and chemical equilibrium.
- Introductory Physical Chemistry II – shorter lecture notes on more advances subjects, such as The Gibbs-Konovalov theorem.
- Physical Chemistry I: Quantum Mechanics – almost 400 pages worth of lecture notes on how quantum mechanics is relevant for physical chemistry. It first looks at some mathematical tools such as matrices and partial differential equations and then stuff like particle in box, angular momentum and tunneling.
- Physical Chemistry II Lecture Notes – an additional set of lecture notes (over 300 pages) on physical chemistry. Deals with equations of state, laws of thermodynamics, thermochemistry, entropy and kinetics.
- Physical Chemistry Course Notes – yet another 300+ page document with lecture notes for physical chemistry.
Free Miscellaneous Chemistry Textbooks
- A Spectroscopy Companion – book on spectroscopy that talks about electrostatics, electrical properties and optical constants.
Physics
Free Introductory Physics Textbooks
- Light and Matter – a physics textbook for individuals with a background in life science. Deals with Newtonian mechanics, conservation laws, electromagnetism, optics and certain aspects of modern physics.
- Simple Nature – calculus-based textbook for introductory physics.
- Conceptual Physics – a physics textbook emphasizing conceptual, rather than mathematical, understanding.
- Introductory modern physics – 80+ course notes on the basics of relativity, quantum mechanics and nuclear physics.
- Motion Mountain – huge textbook on classical mechanics, relativity, quantum mechanics and unification.
Free Classical Mechanics and Mechanics of Materials Textbooks
- Mechanics – a calculus-based textbook on Newtonian mechanics.
- Mechanics – an overview of classical mechanics including topics such as Newton’s laws of motion, one-dimensional motion, force and dampened harmonic oscillators, two-dimensional motion like projectiles, central forces, rotating reference frames, three-dimensional movement and systems of particles.
- Mechanics of Materials – a textbook on the basics of the mechanics of materials. Describes stuff like stress, strain, mechanical properties of materials, axial members, torsions of shafts, symmetric bending of beams, stress transformation, design and failure as well as stability of columns.
- Mechanics Course Notes – looks at basic mechanics, Lagrangian dynamics, linear oscillators, small vibrations and non-inertial reference frames.
- Modern Mechanics – a solid 500+ page lectures notes on various aspects of mechanics, from the ground up (1, 2, 3, 4).
Free General and Special Relativity, Astronomy, Astrophysics and Cosmology Textbooks
- General Relativity – textbook on general relativity for upper-undergraduate level, but excellent for self-study.
- Lecture Notes on General Relativity – the text is called “lecture notes”, but it is more like a 400+ textbook.
- Lecture Notes on Special Relativity – a three-part text on special relativity that covers Minkowski space and Lorentz transformations, length contraction and time dilatation, relativistic kinematics and experimental tests of relativity.
- Einstein Light – mix between textbook and website, but has introductory information on many aspects of relativity.
- Introduction to Differential Geometry and General Relativity –
- Notes on Relativity and Cosmology – textbook on relativity and cosmology by Marlof.
- Elements of Astrophysics – 400+ page textbook on special relativity, dynamics, random fields, properties of electromagnetic radiation, thermal radiation, radiative transfer, radiation fields, classical, quantum and relativistic field theory, inflation, origin of cosmological structure and so on.
- All astrophysical and cosmology coverage in the Review of Particle Physics – a series of reviews on Big Bang cosmology and nucleosynthesis, cosmological parameters, dark matter, cosmic microwave background, cosmic rays, solar neutrinos, neutrino mass, mixing and flavor change.
- Course Notes on Galaxies and the Universe – covers topics such as galaxy classification, global properties and systematics, dust in galaxies, stellar content, star formation, environmental effects on galaxies, dark matter, large-scale structure, galaxy interactions and merges, gamma ray burst and evolution of galaxies, galaxy formation and cosmic microwave background radiation.
- Introduction to Black Hole Theory – an introductory overview on black holes that details topics such as Schwarzschild solution, Chandrasekhar limit, gravitational collapse, Penrose diagrams, four laws of black hole dynamics, Hawking radiation
- Lectures notes on Cosmology and Stellar structure – shorter overview on the expansion of the universe as well as stellar formation and structure.
Free Miscellaneous Physics Textbooks
- Electricity and Magnetism 350+ course notes on electricity and magnetism that covers topics such as electric fields, electrostatics, conductors and isolators, electric potential, magnetic dipole moment, capacitors, resistors and batteries, magnetic force and relativistic field transformations (1, 2, 3).
- Thermodynamics and statistical models in physics – 100+ page course note overview over thermodynamics and entropy.
- Energy – overview of the physics of energy. Talks about kinetic and potential energy, mass-spring oscillators, thermal energy in solid material, the photoelectric effect and quantum mechanics.
- Vibration and Waves Course Notes – almost 300 page course notes about vibration and waves (1, 2, 3).
Mathematics
Free Calculus Textbooks
- Calculus – short introductory textbook for calculus.
- Elementary Calculus: An Infinitesimal Approach – another calculus textbook, but a more rigorous treatment of infinitesimals
- A Brief Introduction to Infinitesimal Calculus – a shorter text focusing on axiomatic formulations.
- Calculus – the first edition of the textbook by Gilbert Strang.
- Mathemtical Analysis I – the basics of mathematical analysis, such as set theory, real numbers, fields, vector and metric spaces, function limits and continuity and differentiation.
- Mathematical Analysis II – a textbook on more difficult concepts of mathematical analysis, such as integration and differentiation on non-linear spaces.
- Vector Calculus – a short 100+ page compendium on vector algebra. Covers topics such as spherical polar coordinates, differentiation of vector functions, partial derivatives, scalar and vector fields, surface integral and flux, the divergence theorem and so on.
Free Linear and Abstract Textbooks
- Linear Algebra in Twenty Five Lectures – a textbook in linear algebra covering topics such as Gaussian elimination, linear transformations, properties of matrices, LU decomposition, determinants and their properties as well as eigenvalues and eigenvectors.
- Elements of Abstract and Linear Algebra – a more difficult and complex textbook on abstract and linear algebra.
- Linear Algebra – another textbook on linear algebra, albeit a bit longer.
Free Probability and Statistics Textbook
- Introduction to Probability – a basic, but long, textbook in probability theory. Covers various probability distributions, combinatorics, conditional probability, expected value and variance, central limit theorem, Markov chains and random walks.
- A Computational Introduction to Number Theory and Algebra – covers basic properties of integers, congruence, computing with large integers, distribution of primes, Abelian groups, rings, finite and discrete probability distributions, probabilistic algorithms, matrices, finite fields,
- Statistics at Square One – introductory statistics textbook by BMJ.
Free Miscellaneous Mathematics Textbook
- Algorithms and Complexity – a textbook covering recursive algorithms, network flow problem and NP completeness.
- Basic Concepts of Mathematics – a textbook covering basics of mathematics, such as sets, number systems, geometry and vector spaces.
Computer Science
Free Cryptography Textbooks
- Lecture Notes on Cryptography – textbook on cryptography covering areas such as trapdoor functions, pseudo-random bit generators, block ciphers, private-key encryption, public-key encryption, message authentication, digital signatures.
- Rethinking Public Key Infrastructures and Digital Certificates; Building in Privac – a textbook on cryptography made available online by MIT Press. Covers preliminaries of cryptography and showing protocols with selective disclosure.
- Handbook of Applied Cryptography – an older textbook that covers the mathematical background of cryptography, public key encryption, digital signatures, hash functions and data integrity, as well as identification and entity authentication.
- Introduction to Modern Cryptography – another free cryptography textbook. Covers block ciphers, pseudorandom functions, hash functions, asymmetric encryption, digital signatures etc.
- Basic Cryptanalysis – introductory textbook on how to break ciphers. Analyze monographic substitution systems, periodic polyalphabetic substitution systems etc.
Free Python Textbooks
- Think Python – solid introductory textbook to the Pytho programming language.
Free TCP/IP Textbooks
- TCP/IP Tutorial and Overview – a 1000+ page detailed look at TCP/IP från IBM Redbooks.