Book #15 in the King and Slater series.
"A vicious and mysterious assassin surfaces in the tiny town of Millinocket, and there’s only one conceivable reason for their presence. As King and Slater watch those they know added to the body count, their world begins to shift: their desire for a normal existence is irreconcilable with the brutal reality of their lives. They’re in Groundhog Day, hunted again and again by malevolent forces, and something has to change."