Not one of her best, but it has some good points. 
 I was confused by who the protagonist was. It was filed as a younger YA book, but one of the viewpoint characters was a thirty-something professor, which seemed like an odd choice and didn't quite fit with the tone of the book or what I expect from Diana Wynne Jones. The other, a teenage runaway with a mysterious past, was much more engaging and I wish the book had been written exclusively from his point of view. Or I could have handled the odd chapter from the professor where it was needed for background information. 
 The usual whimsical sense of humour was present and correct, but the plot itself was a bit lacking, and there was no sense of adventure. There were quite long sections of the book where not much was happening, or whenever something did happen they took a break to go back home for dinner and sleep. When the antagonist or his minions did appear they were brilliantly creepy and the potential was there, but they had too little time in direct conflict with the protagonists. The magic system - the idea of faerie folk spawning human counterparts when they spend too long in one place - was a fantastic idea but didn't have much impact on the plot. Instead, it almost seemed thrown in as an aside. 
 The final resolution felt like a letdown - not the showdown between the two sides but the way the outstanding questions and plot threads were tied up, which didn't have any (or not enough) foundation in everything that had gone before.