The On-Line Books Page <http://onlinebooks.library.upenn.edu/> specializes in creating a list of all books on-line from any source. Searching there is a good place to start.
If you're looking for commercial books, like current textbooks or bestsellers, you're not likely to find them here, since recent books are not in the public domain. For these, you should look for commercial booksellers on the Net--any search engine will direct you to some if you enter search terms like "shop ebook".