We don't have books yet. That's been a common request, so we'll be launching with books by the end of the month. If you'd like, I can notify you as soon as we do.
Please check out the Book Prices API from DataWeave here: http://www.dataweave.in/apis/dataset-Book-Price-Search-By-IS... (Full Disclosure: I am an employee of DataWeave). Currently, we are serving data from Indian eCommerce stores, but expansion to other geographies is in the pipeline. You can search by ISBN as well as many other fields.