Elephant and Piggie Books
Elephant and Piggie are the best! They are written by Mo Willems, who is known for "Don't Let the Pigeon Drive the Bus" and "Knuffle Bunny"--and E&P are just as funny.
They are super ultra easy to read, but sure don't sound like boring easy readers. Elephant (Gerald) and Piggie are best of friends, and are always encountering some hilarious adventure or another.
My favorite is "We Are In a Book", where they keep making the reader (who is reading aloud) say a word, and every time they succeed, they bust up laughing! Gerald's world is shaken when he discovered that all books must end--but they come up with a great solution.
But beware: you may have to get them all. I did!