Unspoken Sin by AJ Estelliam

Detective Mattie Hart thought she was getting closer to the truth.
Instead, she has walked straight into the killer's hands.
Attacked by a figure dressed in black, Mattie is bound and gagged and left completely at the mercy of her unknown captor. She has no idea who has taken her-or what they intend to do to her.
Across town, DI Gray Sable is waiting for Mattie to arrive at her flat. Their friendship has been developing into something deeper, and with Mattie expected at 7.30pm, Gray knows something is wrong when she doesn't show.
But as the hours pass, Gray begins to fear the worst.
Mattie had been investigating the murder of Andrew Sparks, a seventeen-year-old found stabbed, sexually assaulted and hanging from a tree. As she started connecting his death to two earlier cases-the murders of Thomas Ryes and Ethan Wilds, also seventeen-year-old boys who disappeared before being found dead-Mattie realised she may have uncovered something far more sinister than a series of isolated killings.
She is hunting a serial killer.
And now the killer has come for her.
With Mattie held captive and time running out, Gray must follow the trail Mattie left behind and uncover the truth before the person responsible silences her for good.
Can Gray find Mattie before it's too late?
Find out in… Unspoken Sin.