Leonardo Series by Mo Willems

Leonardo is truly a terrible monster--terrible at being a monster that is. No matter how hard he tries, he can't seem to frighten anyone. Determined to succeed, Leonardo sets himself to training and research. Finally, he finds a nervous little boy, and scares the tuna salad out of him! But scaring people isn't quite as satisfying as he thought it would be. Leonardo realizes that he might be a terrible, awful monster--but he could be a really good friend.

One day Sam, the most scaredy-cat kid in the whole world, makes a terrifying discovery. It's not Frankenthaler the monster. It's her friend--Kerry!Kerry, the second most scaredy-cat kid in the whole world, also makes a terrifying discovery. It's not Leonardo the monster. It's his friend--Sam!"AAAAH!" yells Sam."EEEEK!" yells Kerry.Something has to be done. Something BIG. But what?