David Weisner is one of the top children book writers EVER! Or should I say "draw-er". His books have very few words, but his imagination soars. They often start quite simply, but then they take you to a completely other place. You can be very young when you first read them, but even once you get all tall and grey, they will keep you young when you read them.
Flotsam is my current fave, as it features the picture-within-a-picture-within-a-picture principal that I am a 100% sucker for. That said Sector 7 is pretty rocking too, what with it's tale of the clouds total revolution over the bland regime. And it's gonna make a heck of a movie too (cuz I'm making it! -- that is, with quite a bit of help from my friends...)