http://tuebl.ca/ has always been fairly good to me. They may not have everything, but the selection be decent, and they have a connection a 'pirate but not pirate' set of forums to request new books as well. (I do wish they'd stop tryin' to have their rum and drink it too.)
I be also goin' to suggest en.bookfi.org but they appear to be blockaded I do be sad to say… Did not know this until I tried to sail there today.