Butcher & Blackbird by Brynne Weaver

Every serial killer needs a friend.
Every game must have a winner.

When a chance encounter sparks an unlikely bond between rival murderers Sloane and Rowan, the two find something elusive—the friendship of a like-minded, pitch-black soul. From small town West Virginia to upscale California, from downtown Boston to rural Texas, the two hunters collide in an annual game of blood and suffering, one that pits them against the most dangerous monsters in the country. But as their friendship develops into something more, the restless ghosts left in their wake are only a few steps behind, ready to claim more than just their newfound love. Can Rowan and Sloane dig themselves out of a game of graves? Or have they finally met their match?