Press Here, by Herve Tullet, Chronicle Books, 2011. Toddler to Preschool to Early Elementary.
Confession: I didn't actually buy Press Here -- someone bought it for my five-year-old son -- but I would have bought it had I known about it.
When I first looked at the book, I didn't think much of it. Then I read it out loud to my son. We read it at least three times in a row and we only stopped because I had to make dinner.
The book itself is such a delightfully simple concept, and in its very simplicity is its magic.