Mercer Mayer is an American children's author and illustrator. He has published over 300 books, using a wide range of illustrative styles. Mayer is best known for his Little Critter and Little Monster series of books.
I think it's important to remember to visit these books, not just when our kids are toddlers but also when they are emergent readers. It's a great confidence booster for a five year old to pick up a book like this, and read it without help. I always go back to these beginner books for early readers. They are fabulous.
We like how some of the illustrations interact with each other. Also some of the illustrations contain other elements that "belong" to the same letter ("Y" being the most obvious), which could be fun to search for as learning progresses. (Aletheia's review: "ahhh...ehhh...kkggyer" [she finally got the corner in her mouth].)