Free and Open Access Textbooks
Algorithms (by Jeff Erickson)
All About Circuits Textbook
American Institute of Mathematics Approved Open Textbooks
Building Blocks for Theoretical Computer Science
Calculus (by Kalid Azad from Better Explained)
Personal comment:
One of the more beginner-friendly books on learning calculus. I recommend it to give it a chance.
Free Math Textbooks
Free Programming Books
Personal comment:
One of my top starred recommendations.
Free Science Books
Game Programming Patterns
Personal comment:
Recommending this so much when starting out with game development.
How to Design Programs, 2nd Edition
Introduction to Embedded Systems, 2nd Edition (2017, Lee and Seshia)
Invent with Python
Learning JavaScript Design Patterns
Learn You A Haskell
Notes for Professionals
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
Personal comment:
I really like the different programming languages approach with this book.
Open Music Theory
Project Gutenburg
Scientific Programming and Computer Architecture
Structure and Interpretation of Computer Programs, 2nd Edition (HTML version)
The Architecture on Open Source Applications
Personal comment:
Self-explanatory title is self-explanatory, I think.