Green Tea Press
All Green Tea Press books are available under free license that allow readers to copy and distribute the text; they are also free to modify it, which allows them to adapt the book to different needs, and to help develop new material.
O’Reilly Open Books
O’Reilly has published a number of Open Books–books with various forms of “open” copyright–over the years. Here is a list of the open books from O’Reilly: