Overheard at the reference desk (we are not a lending library)...
"Hi, can you check books out from the library?"
"No, but you can sit at one of these tables and read them."
"Oh, they just have numbers on them, so I wondered..."
How silly of us to put numbers on books if we're not going to allow them out of the building. There's no other reason imaginable why we would give them barcodes and/or call numbers. There's not any reason why we would need to keep track of them ourselves.