By Dianna Wynne Jones.
I read it yesterday in between The Bone Doll's Twin and The Waterborn. There is pretty much nothing to say about it, since it's classic. If you haven't read it, you should (unless you detest humor and completely hate life.)
The same goes for the book it's a sequel to, Howl's Moving Castle, which has also been made into a movie in a form which slightly resembles the book. You don't really have to read Howl first, but you'll probably feel smarter and guess more of what's going on in this one.
Ummmmm. And since I mentioned it, I don't think The Bone Doll's Twin is worth reading, really.