Since I've just finished, I must first say that the ending is a wreck. There is not the slightest hint to the twist ending that happens so it appears extremely suddenly, and the 'last battle' is written like the essays I only remembered about 24 hours before they were due: badly. The attempt at showing a peaceful Markham falls flat, and is equally sudden. A mildly interesting character is introduced only for him to be brought literally nowhere (see my updates while reading this). I've read 'fairy tale endings' before, but this is worse than that: it's a badly written one.
In terms of the story, I was experiencing deja vu a lot of the time. A bunch of ghosts who band together to help human friends? Ah yes, The Beasts of Clawstone Castle. The moral that nature and places should be preserved in time? Oh, Monster Mission. The cliche rich, evil villains who want to destroy an important place? The aforementioned two, and The Secret of Platform 13. What would be a fairly interesting plot on its own comes across as a cross-stitch of the author's mother's own works. I understand the plot was a joint effort between them, but this is a bit much.
If you're going to read this book, you will get the most enjoyment out of it if you haven't already read the above mentioned works, or if you've read them so long ago you don't remember them that much. If you have ... well ... You should probably still read it if you like Eva Ibbotson, but be warned that you already know what's going to happen.