We are massive Mo Willems fans in this house. We own almost all of his books (and those we don't already have are on our wish list), we went to see the Don't Let the Pigeon Stay Up Late live show, even taking Joe who was only a few months old at the time (he seemed to enjoy it), Harry even sleeps with a Pigeon soft toy and we bought him the Pigeon Game for his birthday (it's great fun).
And if you've never read any Mo Willems books, you really should. Knuffle Bunny and Knuffle Bunny Too are classics (we haven't yet got the third Knuffle Bunny book but we're hoping Father Christmas might bring it).
The Pigeon Finds a Hot Dog (but they're all great), and Time To Pee not only helped Harry learn when to, you know, pee, it also made him laugh so hard he almost fell off the bed.
The Elephant & Piggie books are hilarious. They're so quick and easy to read (Harry loves deciding if he's going to "be" Elephant or Piggie) and the books all have brilliantly simple and subtle messages about sharing, self-esteem, etc.