Hmmmm... I've hit the following sites for out of copyright books
http://archive.org/details/texts
http://openlibrary.org/
http://gutenberg.org/
http://manybooks.net/
I've downloaded from Baen's free library and purchased others from their website (latest was A. Bertram Chandler's Commodore Grimes books -- I originally read most of them in the form of Ace Doubles which may say something about how long I've been kicking around). (
www.baen.com )
I've also used other sites mainly due to the joys of Calibre being able to do a decent job of converting other formats to .epub. I had a recommendation for a UK site The Book Depository (
www.bookdepository.co.uk ) however I couldn't find any ebooks on their site -- my problem I suspect. One site I found rather disappointing was SmashWords since the formatting and editing on their books was, putting it mildly, rather spotty -- I really don't enjoy editing and reformatting a book before reading it.
I've also scanned in, OCRed and cleaned up a couple of my books. Not a task for the faint of heart.
My daughter downloads quite a few books but I've long since learned not ask my offspring where they obtain anything on the web. Plausible deniability is what I strive for.