Not official and not yet working, but AJAX Search API will soon be supporting Book Search.
http://img178.imageshack.us/img178/4839/eea8.gif
On line 182 of this file*, core of the AJAX Search API, has a function called GbookSearch. As the name suggests, Google Book Search. :)

*http://www.google.com/uds/js/uds_compiled.js

Well-spotted!