Another great place to buy books is BookOutlet. They sell primarily overstocked books, and I'm always pleasantly surprised by their selection. I'll add books to an Amazon wishlist than every few months take that list to BookOutlet and search and they usually have 20-30% of what I'm after for less than half of Amazon's price. The books usually come with a single dot or swipe of a sharpie along one of the open page edges for a reason I'm not totally clear on but must have something to do with them being contractually unable to sell books in perfect condition or something.