This has become one of my favourite Diana Wynne Jones books. There are two beautifull worked out plots which gradually, and complicatedly, merge into an even better single one. The characters as ever are strongly envisaged and the setting wholly real even at its most fantastical. It is full of delights. The city of levels is a tour de force, Romanov's island magical, Roddy's epiphany a ravishing delight, the denoument both deeply serious and very funny.
A glorious book.