As far as I am aware, all of these books are free (licenses that allow you to read these for free). I usually stick with the URL's that have a .edu or other well known sites like O'Reilly.
A First Course in Linear Algebra (A Free Textbook) (Mostly math, but has some links that are for programming)
free-programming-books/free-programming-books.md at master · vhf/free-programming-books · GitHub (Mostly programming, but also includes other things like math)
http://neuroscience.uth.tmc.edu/ (Neuroscience)
ABSTRACT ALGEBRA ON LINE: Contents (Abstract Algebra, advanced type of algebra)
Online Introduction to Biology - The Book (Biology)
Structure and Interpretation of Computer Programs (Programming)
Books by Ivan Marsic (Computers, Programming, Networking)
Online texts (Advanced Math)
The Feynman Lectures on Physics (Famous physics lectures by Richard Feynman)
books (More advanced Math)
Classics in the History of Psychology -- Author Index (Works of famous Psychologists)
CCNBook/Main - Computational Cognitive Neuroscience Wiki (Computational Cognitive Neuroscience)
Chess Tactics Explained in English: Ward Farnsworth's Predator at the Chessboard (Tactics in Chess)
I have a lot more that I will add to the list.
A First Course in Linear Algebra (A Free Textbook) (Mostly math, but has some links that are for programming)
free-programming-books/free-programming-books.md at master · vhf/free-programming-books · GitHub (Mostly programming, but also includes other things like math)
http://neuroscience.uth.tmc.edu/ (Neuroscience)
ABSTRACT ALGEBRA ON LINE: Contents (Abstract Algebra, advanced type of algebra)
Online Introduction to Biology - The Book (Biology)
Structure and Interpretation of Computer Programs (Programming)
Books by Ivan Marsic (Computers, Programming, Networking)
Online texts (Advanced Math)
The Feynman Lectures on Physics (Famous physics lectures by Richard Feynman)
books (More advanced Math)
Classics in the History of Psychology -- Author Index (Works of famous Psychologists)
CCNBook/Main - Computational Cognitive Neuroscience Wiki (Computational Cognitive Neuroscience)
Chess Tactics Explained in English: Ward Farnsworth's Predator at the Chessboard (Tactics in Chess)
I have a lot more that I will add to the list.