Ah. I hadn't seen that problem. One problem I saw is that some are the shorter ISBN, and some are the longer ISBN, but I was assuming that's not a problem to code. I also know that some of the products don't have ISBNs, like monthly webscriptions.
Mostly, I was thinking it might be useful to find out if two entries are for the same book. This is starting to sound like it might not work as well as I'd hoped though.