Open Library API

From their site: One web page for every book ever published. It's a lofty, but achievable, goal. To build it, we need hundreds of millions of book records, a brand new database infrastructure for handling huge amounts of dynamic information, a wiki interface, multi-language support, and people who are willing to contribute their time, effort, and book data. To date, we have gathered about 30 million records (13.4 million are available through the site now), and more are on the way. A project of the Internet Archive.