Google has released the Google Book Search Book Viewability API.
Here's an excerpt from the API home page:
The Google Book Search Book Viewability API enables developers to:
- Link to Books in Google Book Search using ISBNs, LCCNs, and OCLC numbers
- Know whether Google Book Search has a specific title and what the viewability of that title is
- Generate links to a thumbnail of the cover of a book
- Generate links to an informational page about a book
- Generate links to a preview of a book
Read more about it at "Book Info Where You Need It, When You Need It."