Although I know there's a genre called urban fantasy/paranormal/supernatural, I haven't dipped my toes into it beyond a few examples that bleed over into the detective genre. As it happens, both of those were also first books in London-set series: Ben Aaronovich "Rivers of London" series (Midnight Riot / Rivers of London, Moon Over Soho, Whispers Underground) and Christopher Fowler's Bryant and May series (Full Dark House, The Water Room, Seventy-Seven Clocks, etc.). The premise of those two earlier series is that there is a supernatural London that exists alongside our real present-day one, and that special police officers can act to police that world. This book takes that same exact line, as a handful of police form a special unit to take on a powerful supernatural serial killer.
But before that happens, the story opens in media res, with two undercover police detectives within the inner circle of London's gangster lord. There's a rather choppy and confusing opening 40 pages or so, in which their operation is about to be shut down if they can't come up with some concrete information. Then something dramatic occurs, and the undercover officers, their boss, and a police intelligence analyst are put together to figure out what happened. It still takes quite a while for their investigation to get going, and even when it does, the pacing is off and description of the supernatural world isn't always clear. The book might have benefited from a revision or stronger editorial hand on the first third. Eventually, it does start to smooth out and pick up momentum, and in the final fifty pages or so the plot really kicks in.
Unfortunately, although strong in atmosphere and horror elements, the book really falters in characterization. At the start of the story the three policemen seem somewhat interchangeable and it's hard to really form any proper picture of them. Eventually, I ended up viewing them more as types (the angry one, the gay one, the analytical one, the boss), rather than fully realized characters. That really took away from my ability to get drawn into their encounters with the supernatural, and is also why I doubt I'll bother reading any further books in the series. It's not that the book or story was bad per se, but without compelling characters to take you along, it's hard to care that much about rich atmosphere for its own sake. Definitely worth checking out if you're a reader with a strong interest in urban fantasy (or whatever you want to call it) or books set in London, but not one I'd recommend widely.