Project Gutenberg has the largest collection of public domain books. These are the books that out of copyright and available for anyone to use for any purpose. Lots of books from the world’s greatest fiction authors such as Dickens, Joyce, Shakespeare, Mark Twain, Anton Chekov and Edgar Allan Poe fall into this category.
This site is the oldest producer of free ebooks on the Internet. Project Gutenberg is an extremely popular site and now has over 20,000 works. If you are not sure where to start check out their Top 100 page.