Tim Spalding has written a tiny little API to change ISBN10 into ISBN13:
A smart young programmer from a book-related company and I were talking. It turns out that, to validate ISBNs and get back both 10- and 13-digit versions he was submitting ISBNs to Amazon Web Services. That’s like calling NORAD to find out if it’s raining.* Nor did he seem likely to hunt around for an ISBN library for Ruby. After all, what he was doing worked.
So I made a quick, very stupid API, ie. http://www.librarything.com/isbncheck.php?isbn=0765344629