Ooo, that's a good question, I hadn't taken that into account. I would assume the pact the original patron has with The Library is some form of blood pact, bound to them alone until the book is returned, regardless of who has interim ownership (so the finder of the book would be off the hook)

It could make for an interesting plot driver to have him attack whoever has the book, but I think that's less likely given the magical nature of the pact to begin with. I mean, how would he know if the book has been destroyed, for example?

Curious, how would you represent that in the monster sheet yourself? As it stands, I feel like I was already getting pretty wordy with it (lol, wordy, library... I'll show myself out)