The Google Books service enables you to search and navigate material that previously was available only in print form much in the same way you use Google's search engine. Like many of Google's services, Google Books has an API, which allows developers to create their own applications backed by the massive Google Books database.
For PHP developers, the Zend Framework provides access to the Google Books API via the Zend_Gdata component. Using this component you can easily incorporate book-related data into your Zend Framework-powered applications, and even create compelling new applications based around this vast literary trove.
In his PHPBuilder.com article, Jason Gilmore explains how to search Google's books database with the Zend Framework's Zend_Gdata component.