Tried uploading Delicious Library's xml file directly to library
thing. Seems like it was a little too aggressive in that I got 900+
books, many of which I clearly don't own; I think it doesn't know
about the recommendations field. But that's what cElementTree is for...
Ah, much better: "LibraryThing found 161 unique ISBNs. 138 of them are new (not already in your library)." (I entered a small pile of books by looking around the room, and then clicking through suggestions and authors and going "oh, I have that one" :-)
delicious_asin.py is the (entirely trivial) extractor.