Are these copyright free textbooks? I am curious to how this free textbook thing works. There are so many free textbook projects going around the world. Can anyone use the content, reference the content and use it for commercial purpose?
So yes and no, depending on the book.
They've shifted to a Creative Commons licence that achieves much the same thing because the GNU FDL has some awkward requirements from it's history as a software documentation licence from the pre-web era.