Thank you - that actually does give me a little help in conceptualizing what they might be used for, though I still haven't thought of an application for myself (I have about 1500 books and don't always think in book level - lots of tags and lots of series - but most of the time, probably). Though I would love to hear from any others out there who might use them - there must be others, I'm guessing!
Am I right that you can't add a single book to a user category?