Down To Zero: Every end has a beginning. (The Eaters of Light Book 1)

London towards the close of 2018. Mallory Vine and Bob Dario are two Special Investigators with the Unit, a secretive branch of law-enforcement that deals in murder, old cases, cold cases, cases of any temperature between luke-warm and downright chilly. That’s what they deal with every day - and then there’s Assignment 41305, the one both of them would have jumped from a speeding train to avoid, the one concerning Philip De Witte.
Dandyesque, deadly, doomed to hide in the shadows to guard his secret, the assassin De Witte inadvertently links the two detectives to an old academic paper from 1895, to a psychologist with a reputation in tatters and to startling new evidence of the paranormal: initiating a journey of discovery that’s shocking even for them.