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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.
These books are included in the Directory of Open Access Books. This means that they have been peer-reviewed and are of interest to scholars.