To follow up on my earlier post Rules of ISBN
, I discovered today that I've had not one but two problems related to ISBNs on the website and in my database. First, as I described earlier, newly expanded 13-digit ISBNs are coming into use, and though at a glance the ISBN-13 for most books appears to be the traditional ISBN-10 with '987-' tacked on to the front (for US publishers at least), in fact the other difference between the two is that the final digit, the checksum, is different. I hadn't noticed that and had been truncating 13s to make 10s, not realizing that the links to Amazon on the website weren't working because of the wrong checksum digits...
Second, which I just realized today, as far as I can tell Amazon does not yet recognize ISBN-13s at all. Some of the Amazon links I'd generated with the expanded numbers don't work, even though they have the correct checksum digits. I've written to Amazon to ask them about this.
So today I did my best to audit all the ISBNs with possibly incorrect checksum digits, and all the ISBN-13s, and check them against what Amazon says is the ISBN for each title. In practice, whatever they say is what I use. I hope I caught most of them, though if anyone clicks on an Amazon link and gets a 404, please let me know and I'll get it fixed.