Free and Open Access Textbooks
This section offers a collection of free (if not, open which is even better) textbooks for everyone. Most of them are something programming-related but you can see more of them at the "Open Educational Resources" section.
Algorithms (by Jeff Erickson)
It's a good book on algorithms (though not recommended as a first book on data structures and algorithms).
All About Circuits Textbook
An electronics textbook made by the Tony R. Kuphaldt. Contains a set of volumes covering electricity and electronics.
American Institute of Mathematics Approved Open Textbooks
Quite self explanatory. It's a list of approved open textbooks for a variety of mathematics-related branches such as calculus, number theory, logic, and differential equations. You can also see how these books has been approved through their open evaluation criteria that has been put up on their website.
Building Blocks for Theoretical Computer Science
A book that gives you a rudimentary class on theoretical side of computer science. Most of the focus are directed on the formal mathematical side of things with the lessons being set theory, logic, proofs, graphs, and calculus.
Calculus (by Kalid Azad from Better Explained)
A free online primer that tackles on learning calculus with intuition.
Personal comment:
One of the more beginner-friendly books on learning calculus. I recommend it to give it a chance.
Free Math Textbooks
A list of free math textbooks from Open Culture.
Free Programming Books
A GitHub repo by the E-Book Foundation that lists out free programming books.
Personal comment:
One of my top starred recommendations.
Free Science Books
Another GitHub repo by the E-Book Foundation.
Game Programming Patterns
A book of patterns that are commonly found in game development. It's also free to read it online (obviously it is why it's in this list).
Personal comment:
Recommending this so much when starting out with game development.
How to Design Programs, 2nd Edition
A computer science book published by the MIT Press that focuses on designing programs and in hindsight, how to efficiently formulate plans and stick with it until the very completion of your program.
Introduction to Embedded Systems, 2nd Edition (2017, Lee and Seshia)
Invent with Python
A collection of free Python online books authored by Al Sweigart.
Learning JavaScript Design Patterns
A free and open source textbook authored by Addy Osmani. It describes different design patterns that are commonly found written in JavaScript as a bridge to implementing them.
Learn You A Haskell
A free web book that teaches you functional programming through Haskell.
Notes for Professionals
Made by folks at Stack Overflow from the Stack Overflow documentation for variety of languages and despite the title, it is quite helpful for beginners like me, as well.
Personal comment:
This is not much of a recommended resource in the long term since it's based on Stack Overflow's experimental documenation project that has been abandoned recently. Also see the /r/learnprogramming explanation as to why this resource is discouraged. Yeah, take caution when referring with this material. Ultimately, it's better to refer to the official documentation than this one though it's nice while it still lasts.
Open Data Structures
An open content textbook that tackles the implementation and analysis of data structures such as queues, dictionaries, and graphs. Offers versions for different programming languages as well for you to take the book with ease in your favorite programming language.
Personal comment:
I really like the different programming languages approach with this book.
Open Music Theory
A open textbook on college-level music theories and lessons.
Project Gutenburg
An archive site of more than 50,000 publicly available (mostly from public domain) digital books. It contains literary materials for different subjects such as music, science, psychology, and more.
Scientific Programming and Computer Architecture
An open MIT Press web book that focuses on analyzing and discussing the performance of computer programs and the practices of writing fast programs.
Structure and Interpretation of Computer Programs, 2nd Edition (HTML version)
Unofficial (and prettier) version of one of MIT's famous programming book (that can also be found online).
The Architecture on Open Source Applications
A bundle of books that are free to read online.
Personal comment:
Self-explanatory title is self-explanatory, I think.