Looking up book title with ISBN

On the Google API's Console, it mentions a "Courtesy limit: 1,000 queries/day". There is nothing that states what happens if you go over.

No. Google does not specifically limit the numbber of lookups. In the TOS they do use the standard laywerspeak, "Google, at its sole discretion, may limit the amount of Google Books Content it delivers to you. ", but I'm sure if you, "Aren't being evil", as they say, you should be good.

Terms of service: http://code.google.com/apis/books/terms.html (which read a lot like the google maps API TOS).

Take a look at https://sourceforge.net/projects/isbntools/files/latest/download.

Or if you are a developer at https://pypi.python.org/pypi/isbntools.