It wasn't that long ago that every library book had a card, and when you checked something out, your name, or a stamp/imprint of your library card number was placed on that card, when you checked the book back in, the card would be removed from a file, placed back in the book, and the book re-shelved- making it trivial to match to the person... now, you would have to look in each book and see who had checked it out, but still not very private.