(c) by Open Library
In my mind, the best part of that decision is that it paves the way for ongoing contributions to be shared without restriction, which is a key to future data proliferation. — George Oats, Director of the Open Library and member of the Open Knowledge Foundation’s Working Group on Open Bibliographic Data 
The Open Library is an online catalog whose goal is to provide a page on the web for every book ever published. The project began in November 2007 and has been inhaling catalog records from some of the biggest libraries in the world ever since. It has well over 20 million edition records online, provides access to 1.7 million scanned versions of books, and links to external sources like WorldCat and Amazon when relevant. Open Library's secondary goal is to get the user as close to the actual document she is looking for as it can, whether that is a scanned version courtesy of the Internet Archive, or a link to Powell's where she can purchase her own copy.
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.
Open Library's catalog was created by the generous donations of libraries who dedicated their data into the public domain. All its data are available for bulk download in original form through the Internet Archive.
Some of the above text adapted from Open Library's FAQ.