Phantom Tollbooth is another one of my favorites–can’t believe I didn’t mention it! I love the Humbug so much.
And if you like The Wednesday Wars, there’s a sequel called Okay For Now that follows Doug and a new book I haven’t read yet called Just Like That about Meryl Lee.
Glad to meet someone else who’s heard of The Twenty-One Balloons. It seems rather obscure, but it is so good!
Semper ubi sub ubi.