Ex Libris is a "this book I've read" application targeting the Norwegian Facebook community. On Ex Libris you can search for books written in and translated to Norwegian. You can add them to your book list, comment, recommend and rate books.
Coding Ex Libris was fun. It's written in PHP and uses FBML, FBJS, Ajax, PHP MySQL wrapper, CSS The Star Matrix Pre-loaded and Bayesian Rating
As I said, it's targeting the Norwegian Facebook community and hence in Norwegian. You'll find it at http://apps.new.facebook.com/exlibris/