I feel torn in trying to think how to review this book. On the one hand, it was effective in creating a sense of horror and feel of a nightmare. On the other hand, many of the characters behaved in annoying and infuriating ways which seriously marred my enjoyment of the book.
The reader is thrown into the horror of the situation quite quickly, contrasting noticably with some of his other novels like Midnight Sun in which it was built up most gradually over the length of the novel. Although in this book, one derives as much horror from the behaviour of many of the inhabitants of Moonwell as one does from the one that eventually emerges from the cave on the moor.
The problem with building up the tension so quickly in a horror novel is that it then has to be sustained through to the end and that can (and did) get quite exhausting. By the end, I had had enough of characters clambering in the dark, glimpsing horrors and being driven insane with fear. Which I guess is why I tend to prefer horror in the short story form.
So, a mixed experience for me with this one. It did what a horror novel needs to do but not really in the way I like it delivered.