The plot of the book is simple. A little girl thinks there's a monster in the attic above her room. When her parents tell her she's just hearing some mice, she goes up there with cowboy boots and a lasso, captures the monster, and drags him to her parents' room. But before her parents can see him, he escapes back to the attic. (That always happens to me, too.)
Now, just glancing at the book I was worried that she might find it frightening, but just like the Berenstain Bears Ghost of the Forest book which she committed to memory in less than a week, she found this one more curious and engrossing than frightening. Indeed, for the past few days, she's added "Nightmare" to her daily posse of invisible friends, and uttered all kinds of crazy sentences like, "Nightmare is sitting wif us having soup for dinner" or "Nightmare is so sweepy; he just wants to go to bed and go to sweep wif no calling for Daddy Nightmare."