www.feedbooks.net has some good books both free PD and new non-free books and editions of olderPD books that are being sold as well. I'm with the others. It depends on what you're looking for at some sites. You won't find books by living authors at sites like manybooks for example unless the author has made them CC books. And don't forget there are a ton of books in the PD right here at MR. Enough to keep a person occupied with reading them for years.