Check out CalCat

CalCat is a new database from the California State Library that shows the holdings of books, videos, audio, maps, and other materials held by libraries in California.
Some background:

The libraries of California have been sharing cataloging records, books and other library materials through interlibrary loan for many years. Using central storage and software available through OCLC, libraries have been able to copy catalog cards and share digital records. This central storage of data has become the basis of WorldCat, a “union list” or database showing the holding of many libraries throughout the world.
In 2005, the California State Library announced a new project that provides every California public library access to WorldCat, and gathers the data from the libraries of all California libraries into a sub-group known as the “California Libraries Catalog”. A federal grant allows every public library access to both of these databases and provides each library the ability to update their holdings with OCLC.