Are you using the deprecated data API?
With Books API v1 (from Labs) you could use the query
https://www.googleapis.com/books/v1/volumes?q=isbn:<your_isbn_here>
for example
https://www.googleapis.com/books/v1/volumes?q=isbn:0735619670
to query a book by its ISBN.
You may want to look at Googles example code: BooksSample.java