pybook is a Web-based application to keep track of your book collection. Although being web-based, it's single-user and is intended to be installed on your desktop, then accessed from anywhere on your home network or even the Internet.

There is also an iPhone application named iLoveBooks, available on the AppStore, which fully integrates with pybook. This means you can add/modify/etc books on either side and after a quick synchronization over the local network, all changes will be mirrored both on the desktop and the iPhone.

So, take a look at some screenshots and download it. And never, ever, buy a book twice again.