* UI - once someone clicks "Add" you really should remove that item from the suggested list - it's very confusing to still see it.
* Beam search / diversification -- Your system threw like 100 books at me of which I'd read 95 and heard of 2 of the other 3, so it worked for me as a predictor of what I'd read, but not so well for discovery.
I'd be interested in recommendations that pushed me into a new area, or gave me a surprising read. This is easier to do if you have a fairly complete list of what someone's read, I know. But off the top of my head, I'm imagining finding my eigenfriends, then finding books that are either controversial (very wide rating differences amongst my fellow readers) or possibly ghettoized, that is, some portion of similar readers also read this X or Y subject, but not all.
Anyway, thanks, this is fun! Hook up a VLM and let people take pictures of their bookshelf next.