Between needing textbooks at certain times and my distinct love for reading, I've long been on the lookout for the best deals on books. Several years ago, a family member mentioned campusi.com. Rather than being another "we always have the lowest price" site, campusi.com searches numerous other sites (including Half.com, eBay, Wal-Mart, and Amazon.com) and offers the cost of buying from each. Shipping, tax, and total cost are listed on a convenient table. I've been using the site for years and rarely find a lower price online than what they list on their site.
And FWIW, it's not just books--they also have sections for CDs, movies, and several other categories.