Book 28: If You Give a Mouse a Cookie, by Laura Joffe Numeroff and illustrated by Felicia Bond. Toddler to early elementary.
You cannot go wrong with any "If You Give..." book, but the first is my favorite. Numeroff's story is sweet and silly, funny and charming, but what really make this book so well-loved by so many children and adults is the magic that is Felicia Bond's illustration. The whole becomes greater than the parts when an excellent storyteller is teamed with an excellent illustrator.
I love this book, and fortunately, my toddler likes it, so I get to read it quite often.