This was an enjoyable read. What begins as a story of undercover policing among London's organised criminals (tensions in the team, possible betrayal, budget pressures) suddenly takes another - and darker twist - as the chief suspect is killed in a particularly gruesome way (not a spoiler - this is all over the book's blurb). The key officers from Operation Goodfellow (the name itself I think a clue to what's going on) are left investigating the darkness behind the crime, with nothing to help them but their "copper instinct" and the systematic procedures of modern policing.
Cornell then adroitly slides this group of bickering, disparate police officers (and one analyst - who has secrets of her own) into a parallel London, inhabited by wonders and horrors that only they can see (but which they are still defenceless against). As in a number of similar recent books by authors such as China Mieville and Ben Aaronovitch, London comes to the fore, almost turning into a character itself. The story gathers pace, with the original criminal gang almost (but not quite) left behind in the pursuit of a truly horrible villain (yet one we're forced, to a degree, to sympathise with). On the way the reader encounters an extremely polite, though infuriating, talking cat, phantom ships and the most haunted shop in London (which, actually, isn't). All great fun, and as a few mysteries are clearly left unexplained, this book is obviously destined to be the start of a series.
I'm looking forward to more.