Let's see, cash in a library book. Cash being untraceable, hmm... I'm torn. Part of me says "Pocket it! Finders keepers, losers weepers! It's no different from finding a $1 or a fiver on the ground." Another part of me would ask the librarian who checked out the book last. Then the first part says "Well if the librarians called them to ask them if they left cash in their book, of COURSE they'll say say it's theirs, whether it is or not! Come on, take the cash! If they're fool enough to put their money in a library book, then they deserve to never see it again!" And then that side would probably win.