A sequel not only to Howl's Moving Castle but to Castle In the Air as well. It lists only the first on the front cover. Gee, I wonder why?
(I note that the movie does a lot of twists and turns and adding and deleting from the book -- but there are still points in it that I don't think would make sense without my having read the book first.)
But, like Castle In the Air, this book has a different main character: Charmain, suddenly recruited to look after her Great-Uncle Norland's house. Her magician great-uncle. And the bits from the other books float in later, and I think the book is intelligible even if you read it as a stand-alone. (Still, you miss some interesting allusions.)
So we have his house, and the kobolds, and Charmain's attempt to go work for the king, and the lubbock. . . .
I think it's not quite as neat in tying up the plot as the first two, but it's still a fun read.