http://www.fictionwise.com/home.html
http://www.ebookmall.com/
http://www.e-reads.com/index.asp
http://www.diesel-ebooks.com/
Fictionwise is a pain to search, unless you have a title or author. I usually get the book info from Amazon.com or a review, publisher or author site.
http://thebestreviews.com/
Any particular genre you looking for?