Why did the book get upset?

The humor in the joke about the upset book hinges entirely on a classic bit of linguistic trickery: wordplay. Specifically, it's a pun, which takes advantage of a single word having two very different meanings. This kind of joke delights us by making an unexpected connection between seemingly unrelated ideas.

When we say a person "has issues," we mean they're dealing with problems or emotional challenges. But in the world of publishing, an "issue" refers to a specific, sequentially numbered installment of a periodical, like a magazine, journal, or comic book. While standalone books typically have "editions" rather than "issues," the joke playfully stretches that publishing term to apply to a book as if it were a long-running series constantly churning out new installments, each one a fresh "problem" for our poor, fictional book.

The comedy comes from the sudden, silly switch between these two meanings. We imagine the book feeling overwhelmed by its "problems," only to realize the punchline is talking about the numerous editions or releases it has had. It’s a relatable feeling of being swamped, applied to an inanimate object through a clever twist of language.