Friendly Fire by Cari Z.

Elliot McKenzie is the king of reinvention. Five years after losing his job and his lover and almost going to prison, his self-help program, Charmed Life, is more successful than he’d ever dreamed. He thinks he’s put his sordid past firmly behind him, until he starts receiving cryptic threats . . . and realizes it might not be as over as he’d hoped.
Security expert Lennox West has been lost since a deadly skirmish in Afghanistan led to his forced retirement from the Army. His PTSD makes helping his ex raise their daughter a challenge. When his ex’s sister asks him to set her boss up with a security system, Lennox isn’t expecting anyone like Elliot McKenzie—a man who captures his attention and makes him feel relaxed for the first time since leaving the service.
But Elliott is dangerously stubborn. Even as the threats against him escalate, he refuses to involve the police, and Lennox fears that stubbornness could kill him. A battle of wills ensues that brings them closer to each other than either man expected. But if the threats turn real, they might not live long enough to get their future together.