The Detective’s Familiar by TJ Nichols

Walking out on his husband, and fated mate, is the hardest thing wolf shifter Sam Carver has ever done.
Detective Mark Buchan knows he’s screwed things up with his husband, but he never expected Sam to leave. Now there’s a killer targeting witches, and Mark is worried he’s next. He needs Sam’s help one last time.
Sam agrees, but only because if Mark dies, he’ll pine away because of the bond. He hasn’t forgiven Mark, and he won’t forget. And the last thing he wants is to fall in love, again, with the man who broke his heart.
This time Mark will do anything to prove to Sam that he’s worth the trouble. Even surrendering his magic.