Any books that we legally can, and that our volunteers want to work on.
We cannot publish any texts still in copyright without permission. This generally means that our texts are taken from books published pre-1923. (It's more complicated than that, as our Copyright FAQ explains, but 1923 is a good first rule-of-thumb for the U.S.A.)
So you won't find the latest bestsellers or modern computer books here. You will find the classic books from the start of this century and previous centuries, from authors like Shakespeare, Poe, Dante, as well as well-loved favorites like the Sherlock Holmes stories by Sir Arthur Conan Doyle, the Tarzan and Mars books of Edgar Rice Burroughs, Alice's adventures in Wonderland as told by Lewis Carroll, and thousands of others.
These books are chosen by our volunteers. Simply, a volunteer decides that a certain book should be in the archives, obtains the book and does the work necessary to turn it into an e-text. If you're interested in volunteering, see the Volunteers' FAQ at [V.1] below.