These books are great. Mo Willems is a genius in our house! They are funny, just the right length and great for learning readers. My 4 year old can 'read' them all the way through. I say read - it's more like memorise, but thats how they learn isn't it? The illustrations are also hilarious. There is a whole series of them available and new volumes are being released all the time - they don't seem to have caught on in the UK as yet (although you can buy them here), but they are huge in the US.
Mo Willems also writes the Pigeon books - another favourite character in our house. I wasn't impressed when we first borrowed this one from the library but when you've read it a few times it really grows on you, plus the fact that my kids find it side-splittingly funny.
If you like the look of these, check out Mo Willem's website - lots of games, colouring pages and info.