PocketPedia is an app that allows you to search for books (or cds) on Amazon, and then store the results in folders. So, for example, you can create list of "cook books" and then include all your favorite cookbooks. You are then one click away from having all the information about the book -- including reviews -- on your iPhone.
The app itself is extremely easy to use (although it took me a few minutes to figure out how to delete folders), and given the price point (Free) its worth checking out. My one big gripe with it is that there is no way to share these lists with a friend. PocketPedia - if you are listening -- make one of the options of a "collection" the ability to email it to one of your contacts. Also, it would great if there was a web interface, with the information being stored/duplicated in the cloud rather than being tied to a specific physical iPhone.