Library books used to have a little pocket stuck to the back cover, with a lined card inside. If you wanted to borrow a book, you would take out the card, put your name on the next line, and give it to the librarian.  When you gave the book back, the card was put back into the pocket.<p>The system was to keep track of who had borrowed a book, so that they could be tracked down if it was not returned.<p>I remember it being really fun in grade school, to realize I had checked out the same book as an older friend.