For those people who don't want to sign up for a Wikipedia account, it might be nice to offer the functionality offered by this "monobook.js" script:

http://en.wikipedia.org/wiki/User:Lunchboxhero/...

The current behavior in Wikipedia is that if you click on an ISBN, it takes you to a Wikipedia "Special:Booksources" page in which you must scroll through page after page after page of various "find this book at THIS library (or bookseller)" links for places all over the world.

The above change to a monobook.js file allows the user to go directly to a source he or she wishes.

For the purposes of this script, I imagine taking the user to Amazon might be the safest "default" setting, although it might be worthwhile to document via commenting how to change the redirect to something else they desire.