So Vicious by Blake Pierce

FBI Special Agent Faith Bold doesn’t believe she can ever return to the force after the trauma she’s been through. Suffering from past demons, she feels unfit for duty and content to retire—until Turk walks into her life.
Turk, a former Marine Corps dog, wounded in battle, suffers from his own demons. But he never lets it show as he gives everything to Faith to get her back on her feet.
Each is slow to warm up to each other, but when they do, they are inseparable. Each is equally determined to hunt down the demons chasing them, whatever the cost, and to watch each other’s backs—even at the risk of their own life.