Midnight Riot is by Ben Aaronovitch. It's fun boy crime fiction with an urban fantasy spin.
Peter Grant is a rookie cop who's facing a posting as a paper pusher when he sees a ghost near the scene of a crime. While trying to figure out what's going on, he's recruited to be the other member of the London police who investigates supernatural crime by Inspector Nightengale. In doing so, he also become Nightengale's apprentice. Yes, Nightengale is a wizard cop. The ghost he investigates leads to a series of horrific crimes where seemingly normal people commit outrageously brutal acts. The only common link is that the perpetrator's faces collapse immediately afterwards. The plot take a detour to negotiate a gang war between the water gods of the upper and lower Thames, and ends in psychic surgery on the ghostly archeological strata of London history after Peter learns who is causing the attacks.
The elements and world building of this supernatural London are cool and feel nicely multicultural: London's spirits are immigrants as much as London's inhabitants. Peter is engagingly hapless as a sceptically minded rookie plunged into a supernatural landscape, and the secondary characters, Nightengale, Molly: Nightengale's carnivorous maid, Leslie May: Peter's first crush and fellow rookie, Mama Thames, and Beverly Brook: the sexy river sprite whom he also becomes involved with, are all sharp and well realized and fun to watch. The eventual identity of the ghost-murderer is a weird but entertaining twist on the theme of "elemental evil," but the ending got a little muddled and hard to follow, with a few too many scenes. I will pick up the sequel, Moon Over Soho, just for the characters.