You're the Scaredy-Cat - Mercer Mayer
Not the very best of Mayer's offerings, but not a bad one either. Much like Mayer's series of There's a Nightmare... books this one also deals with nighttime fears in a lighthearted way. In this story we have a little boy who inadvertently scares himself with his own ghoulish stories.. Overall a funny story with plenty of engaging and expressive illustrations.

For Mercer Mayer at his very best try the following:

The Wizard Comes to Town
Professor Wormbog in Search for the Zipperump-a-Zoo
Liza Lou and the Yeller Belly Swamp
Herbert the Timid Dragon