Rivers of London

While guarding a gruesome murder scene, Police Constable Peter Grant has a chance encounter with a ghost, changing his career path from pushing paper to a specialized one-man-unit dealing with magical crime. Working to identify the puppeteer behind what turns out to be a string of murders while keeping the Queen's Peace among the supernatural citizenry, he comes to see that not everything is as it seems.
This book is the first volume of Ben Aaronovitch's Peter Grant series and has been published in the US under the title "Midnight Riot".