The Google Book Search Data API allows client applications to view and update Book Search content in the form of Google Data API feeds.
Your client application can use the Book Search Data API to issue full-text searches for books and to retrieve standard book information, ratings, and reviews. You can also access individual users' library collections and public reviews. Finally, your application can submit authenticated requests to enable users to create and modify library collections, ratings, labels, reviews, and other account-specific entities.
For more information on the Book Search Data API, please refer to the official PHP Developer's Guide on code.google.com.