Everything by Mo Willems is awesome, and there's more meta goodness in the Pigeon books as well.

Seconded: I have not read everything by Mo Willems, but everything I have read has been awesome. (My now three-and-almost-a-half-year-old niece seems to feel the same way. We had three Pigeon books in the house and she would demand that I read all of them to her and then read them over again.)