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.
The system was to keep track of who had borrowed a book, so that they could be tracked down if it was not returned.
I remember it being really fun in grade school, to realize I had checked out the same book as an older friend.
"In early 1943, Ulam asked von Neumann to find him a war job. In October, he received an invitation to join an unidentified project near Santa Fe, New Mexico. The letter was signed by Hans Bethe, who had been appointed as leader of the theoretical division of Los Alamos National Laboratory by Robert Oppenheimer, its scientific director. Knowing nothing of the area, he borrowed a New Mexico guide book. On the checkout card, he found the names of his Wisconsin colleagues, Joan Hinton, David Frisch, and Joseph McKibben, all of whom had mysteriously disappeared. This was Ulam's introduction to the Manhattan Project, which was America's wartime effort to create the atomic bomb."
I always enjoyed seeing the names of people who had read a book before me at the university library, and seeing how long it had sat on the shelf between readers. It was like joining a secret club that spanned generations.
The population density is also much less if we scaled the population of my apartment up to 13,000 km², but that isn't how metropolitan areas are traditionally measured. The NYC metro area includes three states, lots of sprawl - so it is much less dense.
I remember the local village library books only had dates on the borrowing slips.
My library card was a 5x5cm card sleeve. If I borrowed something, the return date was stamped in the book, and on the book's removable 5x15cm slip. That slip was put inside my card, and probably ordered by due date in the librarian's index file.
The system was to keep track of who had borrowed a book, so that they could be tracked down if it was not returned.
I remember it being really fun in grade school, to realize I had checked out the same book as an older friend.