An entertaining police procedural with some supernatural shenanigans: 4/5 stars. 
 Fans of "regular" crime fiction shouldn't let the mention of ghosts, vampires and river spirits put them off this book. First and foremost this is the story of crime and the ensuing police investigations. The characters quickly accept the presence of supernatural elements and get on with their normal jobs, most of them slightly miffed that the supernatural elements are making said job trickier. 
 The main character, Peter Grant, is likeable and a good narrator. He's witty and some of his observations are amusing. He's self-depreciating and asks interesting questions which mean his failings as a police officer never become irritating. 
 The descriptions of London are great and the events which take place in central London will be particularly amusing for anyone who has visited, worked or lived in the city. 
 I took a star away from the book because the plot is just ok. The supernatural threat wasn't dramatic enough. The events, while interesting, didn't have me on the edge of my seat, even when the threat was supposedly at its highest. And I'm not sure the feud between the god and goddess of the Thames added to the book. I can only guess it was necessary for "world-building" purposes, laying important foundations for the rest of the series. Perhaps now that's done and Peter's training is coming along we can expect some really nasty villains to deal with in future installments... I'll have to put the rest of the series on my TBR to find out (the average Goodreads ratings do edge upwards as the series continues).