At its heart, Open Library is a catalog. The project began in November 2007 and has been inhaling catalog records from some of the biggest libraries in the world ever since. We have well over 20 million edition records online, provide access to 1.7 million scanned versions of books, and link to external sources like WorldCat and Amazon when we can. The secondary goal is to get you as close to the actual document you're looking for as we can, whether that is a scanned version courtesy of the Internet Archive, or a link to Powell's where you can purchase your own copy.
On a slightly more technical note, apart from that page being an online representation of a library catalog card, Open Library also gives people a URI for a work, an edition or author or other book-ish resource that can be used as a pointer and connector for information about books; a Uniform Resource Identifier indeed.
September 13, 2010