See this SO stream:
https://stackoverflow.com/questions/1297700/what-is-the-most-complete-free-isbn-api
Not. Google does not limit the number of searches. At TOS, they use the standard laywerspeak, โGoogle, in its sole discretion, may limit the amount of Google Books content that it provides to you,โ but I'm sure if you are โNot evil,โ as they say you should be good.
Terms of Service: http://code.google.com/apis/books/terms.html (which is very similar to the Google Maps API API).
cschooley
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