Libraries track books individually. If you and I each check out a copy of War and Peace, and you turn yours in on time but I don’t, they know who to pester.

I think there’s a continuum from the cops being able to say “we found this here book at a crime scene, who had it checked out?”, which should clearly be allowed, to “the security footage shows a copy of War and Peace with the libraries sticker on it in the crooks hand, who had it checked out”, which should probably be allowed, to “we need to know everybody who checked out The Anarchists Cookbook in the last six months because the crook used an IED thats described in there”, which probably should be allowed.