From Henry Petroski’s The Book on the Bookshelf:

Like wrestlers in a ring, two books on an otherwise empty shelf lean against each other in an uneasy stance. Three books are like one basketball player caught between two opponents in a double-team. More books are like a pack of schoolboys, playing Johnny-Ride-a-Pony against the school yard wall. Mostly, however, a shelf-not-full of books is a train full of commuters caught frozen in time leaning against one another in a tenuous balance between gravity and acceleration.