This book is a journey down through the cracks of the structured "normal" world into a nightmarish dark fantasy world that seems to be partially free from regular Newtonian physics. This "neverwhere" reminds me of some of the stuff that shows up in Harry Potter books and Lord of the Rings. One difference with this book is that the author is doing all that's possible to gross out the reader. (e.g. The bad guys eat slugs and bite off the heads of pigeons.)
It's a story of a young adult male representative from the normal "above" world accompanying a sexy looking female pixie resident of the "underside" world who has supernatural powers that allow her to open locked doors. This young pixie happens to be an orphan who's parents were murdered by evil inhabitants of this strange underworld for mysterious reasons. Their quest (ordeal) is to search through this dark and slimy underworld is to solve the mystery of why her parents were murdered.
At another level the story can be seen as a portrait of a troubled mind. I kept wondering if this guy was going to wake up to find it was all a bad dream. One reviewer has suggested that its an image of a Freudian repressed mind. One recurring theme seems to be facing one's worst fears. There is one place in the story that it sounds much as a mind experiencing a psychotic break. There are things suggestive of symbolism or metaphor, some more than suggestive. (e.g. There's a crucifixion, and guess what follows.) The world described here certainly isn't heaven, so it must be the other place. Indeed the story involves the keys to heaven's door that turn out to be the keys to the other place.
I'm a fan of neither science fiction nor fantasy. This is not the sort of book I would choose to read on my own. But one of the benefits of belonging to a book group is that they sometimes select books outside my usual areas of interest. Occasionally this results in my being exposed to a book that I enjoy. Not this time.