The Very Hungry Caterpillar by Eric Carle, 1969.

Concept book in disguise!

A little caterpillar eats too much, gets a tummy ache, then rights his eating habits and turns into a butterfly. The story itself is engaging, but it also teaches the days of the week, counting, one-to-one correspondence (reinforced through the unevenly cut pages and tactile holes), nutrition and metamorphosis. Bright colors and lots of texture in the simple illustrations contrast with white space to make the book all at once straightforward but worth a few extra seconds per page of lingering.

No wonder this was a favorite when I was a child, and a favorite for my own children now. Pie and Fish especially love the page with the cupcake and watermelon and salami and sausage (shaushage, said Fish when he was younger). Of all Carle's books, this is my absolute favorite.