You may have a problem with the Amazon URL that you used to create the buy button, or it could be some sort of incompatibility with something else on your website.
Could you post a link to the book page that should be should be displaying the buy button, and post the Amazon link you have entered for that buy button, so we can see?
@paross, it looks like your theme is not compatible with MyBookTable. It is generating some Javascript that is interfering with the Shadow Box Buy Buttons. You should either turn off the Shadow Box or try using a different theme (I would test to make sure it is working with the built in WordPress themes).
~Tim
