This was originally written by David J.C. Briggs on the Conceptart.org forums. (The original post can be found here.) All credit goes to him for compiling this list, I'm just passing the word on.
The Internet Archive is a massive digital library that stores all sorts of material in the public domain, from music and movies to images, webpages and nearly three million books. All of this is open-source and available to the general public free of charge. The books documented below cover all sorts of subjects from different eras, from anatomy to portraiture, fashion to architecture, from the works of Leonardo Da Vinci to the letters of Vincent van Gogh.
And this is only scratching the surface! If you head over to the Archive and search for whatever subjects catches your interest most you can find much, much more. Just select "Texts" in the search box at the top of the page.