If you like classics, many public domain books are available free on gutenberg.org
http://www.gutenberg.org/browse/authors/a
Or scroll up near the top of this forum page and click where it says E-books to get public domain books which have been uploaded by mobileread members.
You may also want to try the free/bargain forum on mobileread where members post information about books they've found free/on sale.
I keep a wishlist at Google Play books and check it regularly - occasionally I'll find one of the books on my list is on sale.