Yesterday I finished Rivers of London by Ben Aaronovitch. Let me save you some time. Rather than read this review you should a. buy this book, and b. read it.
For those of you still here, please note that I have no compunction in wasting your time. So, onwards.
1. The definition of the genre Urban Fantasy is hotly debated. So when I say that this is urban fantasy, it is also a. set in a city; b has fantastical magical elements; and c. contemporary. In addition it is d. a crime thriller of e. the police procedural type.
2. The supernatural detective novels I have read of recent times seem to be of the gritty noir type. Harry Dresden often seems to be the 21st century magical descendant of a Raymond Chandler or Dashiell Hammett protagonist. More interesting to me are the Felix Castor novels of Mike Carey, which are set in London and are informed by the British crime tradition and, most obviously, by Carey's run on Hellblazer. However one thing that these have in common is that they are very depressing; bad people do bad things and our protagonist has to lie to and betray his friends and put the closest thing to innocents this bad old city has in harms way in order to get the bad guy. It's The Big Sleep with Vampires!
3. Rivers of London is not so depressing; in fact it is exhilarating. Our hero, Peter Grant, is a copper, a member of the Metropolitan Police Service. Having completed his two years probation he doesn't quite have the aptitude to be a theif-taker and is being considered for the department that deals with the paperwork. However, while guarding a murder scene, he is approached by a witness who turns out to be a ghost, and ends up instead in the loose network of those who keep the Queen's Peace amongst the supernatural. As might be expected in a city getting on for two thousand years old, there are a lot of spirits and so forth connected to various places and things. A significant sub-plot involves the titular rivers; the spirits that embody them and a dispute among them. For most of the story Grant finds the magical world aggravating; his A-level science makes him want to look for explanations, and his police training means that most of the time he comes across problems. In the climax however, he discovers/realises that he can make them work for him. If I point out that he begins the final chase from the former site of Bow Street Magistrates Court it comes as no surprise that two hundred and fifty years of policing supports him.
4. The novel mixes actual London with historical, folklore and legendary London in a way I find very appealing. It's similar to the way I see places I know about; I might point out the tiles (indicative of the era of the building) and then tell a story about how the river behind is where Canute beached his ships on his way to Denmark, then go on to explain that he was descended from the god Odin, and tie it all together with a neat bow.
The villain is telegraphed from the first, but is not obvious until the clues pile up and, as becomes clear, is a version of a spirit of London we've seen many times. I will think twice before using the phrase "knock his block off," again.
5. As I've said, it's funny and lighthearted. Yet despite that I can't help noticing that several of Grant's friends are hospitalised in the novel and he sends one away (as part of a deal in order to keep the Queen's Peace). It's almost noirish in it's ending!
Read This: If you are a human being with a fair understanding of English.
Don't Read This: If you don't like police procedurals, London or fantasy. But if so why are you reading this?
Also: The author wrote Dr Who episodes in the late 80s.
 a. and b. are the minimum requirements, but currently marketed urban fantasy often imply c. and sometimes d.
 The only city I've ever lived in. I also have a love-hate relationship with both the city and fictional versions of it. Maybe more blogposts to waste your time on that? We'll see.
 Who then tell him to clear off in the sequels, but always come through in the end. I don't know if they are loyal ("he was your partner and you’re supposed to do something about it") or very stupid, or maybe both. Both probably, and the knowledge that if you don't back him this time, the bad guy will get away and the hero, who is at least half a step better, will die and you'll be sad as you sit alone in the underground bar drinking neat whiskey.
 I've walked down many of the streets that Aaronovitch describes and didn't catch him in a mistake or alteration for the sake of the plot. It's also clear to me that he's walked and driven down them many times in different seasons and at different times of day.