The BEST site is dealoz.com it is an aggregator, sort of the kayak.com for books. It searches Amazon, abebooks, ebay, half, B&N (which actually once came out the cheapest!) and many more. Plus it also lists rentals (which I think are NOT cheaper, being that if you buy books and then sell them it will cost you less than a rental; with all due respect tp Dan). Additionally it will list available coupon codes to each listing which can add to your savings.