All these books are free to read: either they're distributed with a free license, or they're in the public domain.

These GNU Free Documentation License licensed ebooks are free to read - the people who created them want you to read and share them.

The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.