Broken Homes (Rivers of London Series #4) by Ben Aaronovitch

Ben Aaronovitch's bestselling Rivers of London urban fantasy series • “The perfect blend of CSI and Harry Potter.” —io9

A mutilated body in Crawley. A killer on the loose. The prime suspect is one Robert Weil, possibly an associate of the twisted wizard known as the Faceless Man. Or maybe just a garden-variety serial killer. 

Before apprentice wizard and Police Constable Peter Grant can even get his head 'round the case, two more are dropped in his lap: a town planner has gone under a tube train, and there's a stolen grimoire for Grant to track down. 

So far, so London. 

But then Peter gets word of something very odd happening on a housing estate designed by a nutter, built by charlatans, and inhabited by the truly desperate. 

Is there a connection?

And if there is, why oh why did it have to be South of the River—in the jurisdiction of some pretty prickly local river spirits?