I recently discovered book price search engines, which retreive prices from a number of different online book retailers and list them for you in ascending order. This has saved me quite a bit of money on books.
There is a variation in the prices returned, because not all search services search the same sites. I did a comparison between some of the many of book price search services and best best ones appeared to be BooksPrice and CampusI; sometimes BooksPrice was cheaper and sometimes CampusI was cheaper. It seems worthwhile to search both.
Price engines have the potential to commodify the market they operate in. Price search engines can reduce search costs considerably, making the market more competitive. The search engines are unlikely to capture the rents that book retailers reap because it will always be simple to go to the online retailer directly.
I think the reason that price search engines can work well here is because books have ID numbers (ISBN), which makes it easy to organize content from many different sites. Other markets, where products are not well identified probably cannot be commodified easily.
One negative impact of commodifying the online book retail market is that it would probably reduce the incentive for Amazon and similar sites to produce book reviews and other useful information about books.