Below is a loosely-categorized collection of links to CS textbooks in a variety of areas that are freely available online, usually because they are one of the following:

- An open textbook (such as PLAI, SF, or the HoTT book)
- An older book that is out of print, for which the copyright has returned to the original author(s) (such as TTFP)
- An authorâ€™s own preprint or draft of a textbook. This includes cases where the author has made special arrangements with a publisher to host an electronic copy of a published text on their homepage while it remains in print.

I also include below a list of papers I consider good stand-alone introductions to certain topics, and a list of links to thorough special topics courses.

If you find one of the links below is broken or has moved, feel free to let me know.

Those with time to spare and looking to have less of it may enjoy browsing the QA call numbers in UPennâ€™s extensive listing of online books. Most of those listed here were found independently over the years, but Iâ€™ve just now (June 2020) learned of this excellent repository of links. Iâ€™ll add to the links below as I find promising books.

Key:

- + = A book I have spent considerable time with and highly recommend.
- No prefix: a book Iâ€™ve perused casually, and looks good, but I have not (yet) spent much time with.

*Note*: These categories have grown quite unevenly over time, and in a rather ad hoc fashion. Iâ€™m planning to overhaul the categorization here in the near future.

Topics such as semantics, types, abstract interpretation, proof assistantsâ€¦

- Software Foundations
- Practical Foundations for Programming Languages (draft)
- Programming in Martin-Lofâ€™s Type Theory
- Proofs and Types
- Practical Foundations of Mathematics
- +Programming Languages: Application and Interpretation (second edition in progress)
- Semantics with Applications
- Lectures on the Curry Howard Isomorphism (draft)
- +Type Theory and Functional Programming
- +Certified Programming with Dependent Types
- Isabelle/HOL: A Proof Assistant for Higher-Order Logic
- Communicating Sequential Processes
- The Craft of Programming
- Denotational Semantics: A Methodology for Language Development
- +Lambda Calculi with Types
- Lambda Calculus with Types
- Logics and Type Systems
- Programming from Specifications
- Using Z: Specification, Refinement, and Proof
- Homotopy Type Theory
- Partial Evaluation and Automatic Program Generation
- Implementing Mathematics with The Nuprl Proof Development System
- Intuitionistic Type Theory
- Concrete Semantics
- PX: A Comuptational Logic
- Specifying Systems: The TLA+ Language and Tools for Hardware and Software Engineers
- +The Essence of ML Type Inference (Manuscript, Chapter 10 of ATTAPL)
- Static Program Analysis
- Principles of Programming Languages
- Alias Analysis for Object-Oriented Programsâ€ť
(Manuscript, chapter of
Aliasing in Object-Oriented Programming ) - Pointer Analysis
- Tree Automata Techniques and Applications
- Modeling Reactive Systems with Statecharts: The STATEMATE Approach
- +Program Logics for Certified Compilers
- Program = Proof
- Foundations of Probabilistic Programming
- Transductions and Context-Free Languages (first four chapters)
- Automata, Computability, and Complexity: Theory and Applications
- An Introduction to Probabilistic Programming
- Graph Transformation for Software Engineers
- Introduction to Compilers and Language Design
- Tree Automata
- Program = Proof
- Counterexamples in Type Systems

This portion excludes category theory, which is now its own section below.

- A Modern Formal Logic Primer
- Natural Logic (pdf)
- Algebraic Topology
- A Concise Course in Algebraic Topology
- Model Theory
- Set Theory
- An Introduction to Substructural Logics
- Mathematical Modal Logic: A View of its Evolution
- Normalization, Cut-Elimination, and the Theory of Proofs
- Logics of Time and Computation
- Mathematics of Modality
- Modal Logic and Process Algebra: A Bisimulation Perspective
- Lectures on Linear Logic
- Dynamics, Polarity, and Quantification
- Non-Well-Founded Sets
- Logic in Action
- Varieties of Lattices
- Tools and Techniques in Modal Logic (the page is in German; the download link is the actual PDF)
- +An Invitation to General Algebra and Universal Constructions
- A Concise Course in Algebraic Topology (near the bottom)
- Logic and Automata: History and Perspectives
- Topology: A Categorical Approach
- +Nine Chapters on the Semigroup Art
- Mathematics for Computer Science

- Site Reliability Engineering
- The Site Reliability Workbook
- The Architecture of Open Source Applications project books:
- Machine Learning in Production
- Explainable AI for Software Engineering
- Object-Oriented Software Construction

- Abstract and Concrete Categories: The Joy of Cats
- Computational Category Theory
- Toposes, Triples, and Theories
- +Topoi: The Categorical Analysis of Logic (also at Project Euclid)
- Categories, Types, and Structures
- Higher Operads, Higher Categories
- Category Theory for Computing Science [pdf] (Barrâ€™s publications)
- *-Autonomous Categories
- Categorical Logic
- Abelian Categories
- Metric Spaces, Generalized Logic, and Closed Categories
- An introduction to fibrations, topos theory, the effective topos and modest sets
- Categorical Homotopy Theory
- Category Theory for Scientists (Manuscript)
- Categorical Algebra (technically a 1965 journal article from Bull. Amer. Math. Soc., 17(1):40â€“106, but up on Project Euclid.
- Higher Topos Theory: Author version, arXiv version
- Basic Concepts of Enriched Category Theory
- The online journal Theory and Applications of Categories archives electronic versions of some out of print category theory textbooks: Reprints in Theory and Applications of Categories
- Category Theory for Programmers
- Higher Categories and Homotopical Algebra author preprint
- 2-Dimensional Categories
- First-Order Categorical Logic
- Generic Figures and Their Glueings: A Constructive Approach to Functor Categories
- +Category Theory in Context
- Higher Categories and Homotopical Algebra

- Implementing Functional Languages: A Tutorial
- The Implementation of Functional Programming Languages
- Shared Source CLI 2.0 Internals (shelved draft)
- Smalltalk-80: The Language and its Implementation (a.k.a. â€śThe Blue Bookâ€ť)
- The Design and Implementation of Probabilistic Programming Languages

- +A Commentary on the Sixth Edition UNIX Operating System (and UNIX v6 source code)
- Distributed Systems, 3rd Edition by van Steen and Tanenbaum
- Linkers and Loaders
- Concurrency Control and Recovery in Database Systems
- Is Parallel Programming Hard, And, If So, What Can You Do About It?
- Operating Systems: Three Easy Pieces
- The TCP/IP Guide
- Notes on Theory of Distributed Systems, CPSC 465/565 (technically notes, but in practice an excellent textbook)
- +Practical File System Design with the Be File System
- Principles of Computer System Design (Authors share second half online, first half published in print by Elsevier)
- Foundations of Databases
- Security Engineering: A Guide to Building Dependable Distributed Systems
- Computer Networking
- Recent Advances in Networking, Volume 1
- Performance Evaluation Of Computer And Communication Systems
- The Network Calculus Book
- +Computer Networking: A Systems Approach
- Software Security: Principles, Policies, and Protection
- Principles of Electronic Devices
- NetBSD Internals
- Open Distributed Systems
- Foundations of Distributed Consensus and Blockchains
- A collection of books on Illumos internals and systems

This section will be split into more categories soon.

- Foundations of Computer Science
- Linear Programming: Foundations and Extensions
- The Design of Approximation Algorithms
- Mining of Massive Datasets
- Model-Based Machine Learning
- Foundations of Data Science
- Think Data Structures
- Think Bayes: Bayesian Statistics Made Simple
- Think Stats
- Algorithmic Game Theory
- This oneâ€™s a bit awkward to find: Click on the â€śResourcesâ€ť tab (right of Description and Contents), and in the resulting â€śGeneral Resourcesâ€ť side-bar that appears to the left, click on the book title. In the table that appears, click where the book title appears under the â€śNameâ€ť column; this is the full PDF of the book.

- Reinforcement Learning: An Introduction (also, direct PDF of author preprint of 2nd edition here)
- Reinforcement Learning: An Introduction, 2nd Edition
- Machine Learning for Data Streams: with Practical Examples in MOA
- Fairness and Machine Learning
- Deep Learning
- Algorithms Illuminated
- Seeing Theory: A Visual Introduction to Probability and Statistics
- Mathematics for Machine Learning
- The Joy of Cryptography
- Cryptography, An Introduction
- A Graduate Course in Applied Cryptography
- Neural Networks and Deep Learning
- Learn Quantum Computing using Qiskit
- Multiagent Systems: Algorithmic, Game-Theoretic, and Logical Foundations
- Dive into Deep Learning
- Discrete Mathematics: An Open Introduction, 3rd Edition
- Models Of Computation
- Graph Theory
- +Computational Complexity: A Modern Approach (draft)
- The Algorithmic Foundations of Differential Privacy
- Grammatical Inference: Learning Automata and Grammars
- Algorithms (by Jeff Erickson)
- PATTERNS, PREDICTIONS, AND ACTIONS: A story about machine learning
- Introduction to Theoretical Computer Science
- Machine Learning: A Probabilistic Perspective
- Probabilistic Machine Learning: An Introduction
- Probability for Data Science
- Julia Data Science
- The Principles of Deep Learning Theory
- Neural Approaches to Conversational Information Retrieval
- The Mathematical Engineering of Deep Learning
- High Dimensional Probability
- Connecting Discrete Mathematics and Computer Science
- Programming Differential Privacy
- Differential Privacy for Databases
- The Principles of Deep Learning Theory

- Introduction to Applied Linear Algebra: Vectors, Matrices, and Least Squares
- The Causal Inference Book
- An Introduction to Statistical Learning, with Applications in R (both 1st and 2nd editions)
- The Elements of Statistical Learning: Data Mining, Inference, and Prediction
- Linear Algebra Abridged (Linear Algebra Done Right, minus proofs)
- Statistical Thinking for the 21st Century
- Convex Optimization
- An Introduction to Probability Theory and its Applications
- [Numerical Linear Algebra] (https://people.maths.ox.ac.uk/trefethen/text.html)
- Linear Algebra Done Wrong

- The Mathematics of Language (via authorâ€™s homepage)
- The Syntactic Process
- Speech and Language Processing
- Natural Language Processing
- Text Mining with R
- Natural Language Processing with Python: Analyzing Text with the Natural Language Toolkit (NLTK)
- Grammatical theory: From transformational grammar to constraint-based approaches. Fourth revised and extended edition
- This is itself an English translation of Grammatiktheorie, which is in German, and also freely available

- Modification
- Representation and Inference for Natural Language
- Natural Language Processing Techniques in Prolog
- Syntax in the Treetops
- Type-Logical Syntax
- Essentials of Linguistics, 2nd Edition
- Indefinite Pronouns (alternate)
- Inquisitive Semantics
- Semantics and Morphosyntactic Variation: Qualities and the Grammar of Property Concepts
- Analyzing Meaning, 2nd Edition

- Topics in Parallel and Distributed Computing: Introducing Concurrency in Undergraduate Courses (preprint)
- Critically Conscious Computing

- Structure and Interpretation of Computer Programs (also: Unofficial PDF Ebook Reformatting, and SICP in Clojure)
- How to Design Classes (draft)
- How to Design Programs (2nd Edition)
- How to Design Programs (1st Edition)
- A Data-Centric Introduction to Computing
- A Functional Introduction To Computer Science (Part I)

While most of the books on this page are focused on more theoretical/foundational topics, sometimes you just need to learn a programming language or look at some reference on a particular language feature.

- Scala
- Programming in Scala (first edition)
- Creative Scala
- Scala Book

- OCaml
- Haskell
- +Real World Haskell
- Parallel and Concurrent Programming in Haskell (on Oâ€™Reilly Safari, but can be read without account or subscription)
- Learn You A Haskell
- Developing Web Applications with Haskell and Yesod

- Standard ML
- Scheme and LISP
- Practical Common Lisp (and PCL in Clojure)
- On Lisp (and one two three ports to Clojure)
- The Scheme Programming Language

- Erlang
- JavaScript
- Go
- Java
- Python
- Rust
- Ruby
- F#
- Prolog

Some of the books listed in other categories are appropriate for someone looking to get started with programming and/or computer science. Iâ€™ve duplicated those links in this section so theyâ€™re easier to find if thatâ€™s what you need.

- Structure and Interpretation of Computer Programs
- How to Design Programs (2nd Edition)
- How to Design Programs (1st Edition)
- Creative Scala

I think the term â€śsoft skillsâ€ť downplays how important these skills are and how difficult they are to build, but Iâ€™m settling for scare quotes because I canâ€™t think of a better concise topic name people will recognize.

- Abstract Interpretation (Classic): Abstract Interpretation: A Unified Lattice Model for Static Analysis of Programs by Construction or Approximation of Fixpoints. Cousot and Cousot, POPLâ€™77.
- Abstract Interpretation (Semantic): Abstracting Abstract Machines. Van Horn and Might, ICFPâ€™10.
- Denotational Semantics: The Denotational Semantics of Programming Languages. Tennent, CACM 19(8), 1976.
- Program Synthesis: Dimensions in Program Synthesis. Gulwani, PPDPâ€™10.
- Type Theory: Introduction to Type Theory. Herman Geuvers, notes from lecture given at Language Engineering and Rigorous
- Software Development run of ALFA summer school, 2009.

- Matt Mightâ€™s Grad Student Recommendations
- Various Books on Category Theory
- Benjamin Pierceâ€™s Great Works in Programming Languages collection
- Karl Craryâ€™s Classic Papers in Programming Languages and Logic course
- Patrick Cousotâ€™s Abstract Interpretation course (readings section)
- Matthias Felleisenâ€™s History of Programming Languages course readings
- Viktor Vafeiadis and Derek Dreyerâ€™s course on Concurrent Program Logics
- Frank Pfenningâ€™s course on Linear Logic
- Michael Shulmanâ€™s seminar on Homotopy Type Theory
- Bob Harperâ€™s course (with videos) on Homotopy Type Theory