Good writers usually place the specific subject of the essay in relation to one or more of the central points of the book. Doing this shows that you are thoughtful.
Also, a thoughtful essay often challenges something that a quick reader of the book might think. For example, "The frequent jokes in the play lead many readers to think it is a comedy. But the jokes among the main characters hide a pervasive sadness that affects almost everything they do, which makes this play a tragedy."