'Nightside' was supposed to be an urban fantasy, leaning farther into horror, about John Taylor...a private eye with...a 'private eye' as his magic power. That gives you an idea of the quality and creativity you'll experience throughout the book. John is pulled back into the shadow city (the Nightside) within London in service of a case after 5 years away and vowing never to return to his place of birth.
Its kind of hard to fully miss these days with urban fantasy, but this manages to do so. The Nightside (which, as others have said is a name/phrase we see ENTIRELY to frequently throughout the book) feels like a thinly veiled and poor shadow of the London of Neverwhere. In fact, the book as a whole feels like an artless imitation of Neil Gaimen's work mashed up with more mainstream urban fantasy, traditional hard-boiled detective novels without a solid grounding in the tropes of genre, and horror minus anything actually disturbing or horrific. It frequently tells rather than shows, with long exposition between characters info-dumping descriptions and information on one another rather than letting us experience the world. While some nods to Zelazny's Amber and Moorcock's Champion are fun references, they push the feeling of the overall book further into realm of low-quality fan fiction. As does the frequent shifting of tone from what is clearly attempting to be horrific/disturbing to a humorous tone that never actually manages to be funny.