The story is about a rookie policeman in modern London, who reveals some unexpected talents and ends up joining the almost defunct magic division of the Metropolitan Police. Some of the deaths in the story are quite gory, but I didn't find the violence gratuitous, since the characters react to the gruesomeness in a suitably appalled way. (Except possibly for a cheerful pathologist - although that may just come with the job.)
What I really enjoyed about the book is the witty way it's written and the excellent portrayal of London. I was hooked on the book after the first paragraph, which contains this gem:
Being a seasoned Londoner, Martin gave the body the 'London once-over' - a quick glance to determine whether this was a drunk, a crazy or a human being in distress. The fact that it was entirely possible for someone to be all three simultaneously is why good-Samaritanism in London is considered an extreme sport - like base-jumping or crocodile-wrestling
Once I'd finished, I wanted to read another in the series immediately and luckily one is published today, and another is on the way.
P.S. In America, for some reason, it's been published as 'Midnight Riot', which title has very little to do with the content of the book - unlike the UK one.