There's a Nightmare in My Closet (There's Something in My Room Series) by Mercer Mayer

The pesky nightmare in the closet won't stay in there at night, even with the door closed all the way. When he gets out, what do you do? Comfort him and put him to bed, of course! The ultimate afraid-of-the-dark story has helped generations of kids deal with the nightmares in their own closets.A perennial favorite.