Why did the book go to the hospital?

This clever little riddle hinges entirely on the wonderful world of wordplay, specifically a pun. The setup immediately makes our brains jump to a living creature needing medical attention, thanks to the mention of a hospital. We’re primed to think about human or animal anatomy and the serious nature of a medical emergency.

The genius of the punchline lies in its ability to take a common medical ailment – a broken spine, which is a very serious matter for us fleshy beings – and apply it literally to an inanimate object. Books, of course, have a "spine" too, the sturdy edge that holds their pages together. When a book's binding gives out, it's not a medical emergency, but it certainly has a "broken spine" in the structural sense. This juxtaposition of a grave human injury with a book's physical damage is where the chuckle comes from, making us appreciate the dual meaning of a single word. It's a classic example of how language can be twisted for a quick laugh, playing on our expectations and the literal interpretations of words.