There's a Nightmare in My Closet by Mayer, Mercer (1968) Hardcover

A little boy anxiously awaits a nightmare he knows lives in his closet. when it finally appears, the little boy shoots it with his popgun. Looking more sheepish than terrifying, the nightmare starts to blubber, and the little boy realizes the nightmare isn't scary at all. He takes the nightmare by the hand and tucks it into bed. Silly and comforting, this is the perfect book to get rid of those bedtime bogeys.