Charmain Baker gets sent, against her will, to look after the house of her Great-Uncle William, who is also the Royal Wizard of Norland, while he's away being cured of illness. The house turns out to have all kinds of dimensions not immediately obvious to the naked eye, but there are problems from rebellious kobolds and a dangerous lubbock, as well as difficulties for the Kingdom of Norland, which is very nearly bankrupt.
(Random aside: can I just say how distracting the lubbock was to me? So far as I can determine, that's not anything from folklore. And I associate the name with a rather dreary city in Texas, known to me mostly because a) it's where we stopped for lunch on road trips to Arizona, and b) it's the home of Texas Tech University, from whence came the various correspondence courses I did in high school. So yeah, that's what I kept thinking about.)
Like Castle in the Air, this is less a direct sequel, more a related book. Howl, Sophie, and Calcifer appear (and their influence is more apparent than in Castle), but mostly they're there to facilitate someone else's story -- in this case, Charmain's.
Alas -- moving into spoiler territory, now -- I don't find Charmain half so engaging as either Abdullah or Sophie before. The problem is that Charmain doesn't want anything, really. Her primary wish is to be left alone to read a book. While I can sympathize with this -- it's a fairly apt description of my own feelings at her age -- it's essentially a passive desire, a wish not to engage with the plot. As a result, stuff happens largely because of random chance, or other characters' actions. Charmain neither has nor contributes much in the way of forward momentum.
That lack of desire means she's largely disconnected from most of the story's content, even when I the reader am interested in it. Whyyyyy does she not want to explore the house?!!! I'm a big reader, but dude, that's enough to distract me from a book. And she fiddles with magic -- despite being raised to think it's disreputable -- but pretty much drops it after one use. Even if you grow up surrounded by that kind of thing, I find it hard to swallow that you'd show that little interest. The only thing Charmain shows any interest in (besides her own comfort and convenience) is helping out in the King's library, but that falls short for two reasons: first, she achieves it without much effort, and second, her passion for that is more stated than felt.
Which is not to say there aren't elements I enjoy. The house itself is cool, even if it doesn't feature as centrally as the title makes me wish, and I like the practicality of the royals of Norland. I also enjoy what we see of the series characters, especially Calcifer's faceoff with the lubbock. Heck, I even enjoy Howl here, though I sympathize with Sophie's desire to brain him with something heavy for the whole "Twinkle" thing. Filter out the lisp, and I like watching him be competent while also managing to entertain himself hugely. :-)
In the end, though, the pieces don't quite hang together as a strong whole, largely because Charmain's not a very effective center of gravity. (I think I actually would have preferred Peter as a protagonist.) It's a bit of a shame; I would have liked more.
Derkholm series next!