Kepler, Lars. Killer instinct series

"This is a Borzoi book"

"Three years ago, Saga Bauer received a postcard with a threatening message. It mentioned a gun and nine white bullets-one of which was intended for Detective Joona Linna. According to the anonymous sender, Saga was the only person who could save him. But as time passed, the threat faded. That is, until now. A sack with a decomposing body has been found tied to a tree in the forest, and a milky white bullet casing turns up at the scene. Soon, the police begin to receive complicated riddles from the killer, taunting them with opportunities to stop further murders. Joona Linna and Saga Bauer struggle to solve the riddles and save further victims before it's too late. As the hunt becomes increasingly desperate, it appears that the killer is unstoppable... and that Linna and Bauer may already be caught in the killer's web"--