In a recent tutorial from the IBM developerWorks website they show you how to link your site with the Google Book search and parse the results with the Zend Framework's Zend_GData component.

This article will introduce you to the Google Book Search Data API, showing you how to integrate and use book search results with a custom PHP application. It includes examples of searching for books by keyword, by language or by author; retrieving book data (including ISBN numbers and thumbnail images); and adding reviews and ratings to books that are already in the database.

They start with an overview of how the search feeds are structured so you can get an idea of whether or not the returned information looks correct or not. From there they get into the code - parsing those feeds, sending search requests based on input strings, getting details on a specific book and pulling in the contents of a user's library.