[–] ardvarcus 0 points 4 points (+4|-0) ago 

Project Gutenberg Australia is the best, because it carries many books still listed as under copyright in the US. Another prime source for public domain books is Google Books.

[–] plankO 0 points 2 points (+2|-0) ago 

I found a fascinating history of the byzantine empire book on project Gutenberg. It's really a great source