When I forgot to return a library book once (or rather I lost it and couldn't be arsed to own up to it), I got a letter from the council a few months later to say that the fines had reached the level of the value of replacing the book (about £5 I think), so if I came down to the library to pay that amount, they'd waive any fines and I could keep the book. I did so and of course just a few days later, the book turned up in a pile of junk I swore I'd been through when looking for it!