Here's a fun prompt: name a children's book that makes you nostalgic. Just one? I could list a lot: Curious George, the Berenstain Bears, Mitz Petel (about a lion and a giraffe wondering how the mysterious Mitz Petel [Rapberry Juice] looks), Dira Le'haskir (apartment for rent), Clifford the Dog, An American Tail (which I first knew in book form - I'm not sure if I ever saw the movie), the Arthur books, Tell Me a Mitzi (with the ugliest human beings in children's literature) and on and on and on.
But I had to pick just one, and "Miss Nelson is Missing" by Harry Allard and James Marshall is it.
I loved this book, in which a substitute teacher that looks like a witch makes the students appreciate their regular teacher, Miss Nelson. At the end of the book it is revealed that Viola Swamp, the substitute, was actually Miss Nelson in disguise.