Endgame - Sarah Barrie

Vigilante turned cop Lexi Winter is drawn into the investigation of a chain of apparently random murders. As the bodies pile up, Lexi realises she is a pawn in a psychotic - and deadly - game.
A breathtakingly fast-paced and compulsively gripping crime thriller from a rising star.

Lexi Winter is studying for her detective exams but her nemesis Vaughn is out there somewhere and her concentration is fractured. Just as she's wondering if her hypervigilance is tipping over into madness, Vaughn resurfaces and issues a challenge. Five people will be dead by the end of the month. If Lexi can identify and save the last one before the deadline, he'll turn himself in.
As Lexi's colleagues scramble to untangle what it all means, Lexi is implicated deeper and deeper into the murders. On top of this, she begins to see a strange figure everywhere she goes. Branded as suffering from PTSD, Lexi's credibility is in tatters with her colleagues, but Lexi knows it isn't her mind that is the problem. A wicked game is afoot, and Lexi is just a chess piece in play - and she suspects that check mate will mean her death. There is only one thing that Lexi can do - play along, wait for the chance to strike back, and bring about the endgame...
This is the fourth and final book in the outstanding Lexi Winter series.
'Phenomenal. Lexi Winter is the gritty hero I didn't know I needed ... I'm now desperate for more' - bestselling Australian author Nicola Moriarty