The Puzzle of Mr. Penumbra’s 24-hour Bookstore

Mr. Penumbra’s Bookstore is an impossibly narrow shop that has shelving that goes from the floor to ceiling three stories high with only ladders to retrieve the books at the top. The used books it sells at the front of the store are a mix of classics with a few popular titles thrown in, but it is the books from the back of the store that are different, even obscure with unknown authors and titles. The customers who use the books from the back of the store never buy, the books only borrow them…