I frequently buy books online, and of course I like getting the best price possible. One of my favorite sites for online book shopping is AddAll.com. You can search by Author, Title, ISBN, or Keyword, and AddAll searches dozens of online bookstores (new and used books from Amazon, Half.com, Walmart, Barnes and Noble, eBay, etc.) for the absolute best price. The best part about AddAll is that it gives you a price list along with the shipping cost from each website for that particular book.
If you love books like I do, you'll love this site!