Project Gutenberg features some of the world's greatest literature, and they also feature older works where the U.S. copyright has expired. You can check for books you need on the Open Education Network, which provides a free library of textbooks anyone can use. The community behind this network focuses their efforts on sourcing academic textbooks that are openly licensed. The majority of books on the network are peer reviewed, and all are entirely free. You can use the network to browse textbooks and subjects, and all books can be downloaded and distributed at no cost.
