are you looking for like... novels.. or school books?
If its novels, used books on amazon are probably your best bet... most paperbacks can be had for a dollar or less... not to bad to take a hit on if some deadbeat doesn't ship.
If its school books and you have some time, ebay is actually a good place to go for some books. I had a friend who bought 4 books for one quarter for a total of like, $120... sold them back to the bookstore when he was done for $180. Talk about awesome... get paid $60 to use books