At last, the piece settles in a bit. While the boys move around, they at least stay in a single empire, giving Christopher and us a chance to explore it a bit. The result, and the fact that there's a resolution at the end of the boo, rather than simply a stopping point, are more satisfying than the previous volume.
At the same time, Christopher indulges in a bit of hocus pocus toward the end - a resolution that has fun elements, but doesn't really have much foundation in the story. It's an adequate ending, but no more. If you haven't read Christopher, don't start here. If you have, I still urge you to read his other, better books before coming to this trilogy.