The Power of Three (Rex Dalton K9 Thrillers Book 1) by JC Ryan

Rex Dalton was sent to Afghanistan to track the heroin trade. Instead, he uncovered a conspiracy reaching to the top of the CIA—and his whole team was killed for it. Now officially dead, Rex is hunting the three men who signed his death warrant. He has no agency, no backup, and no name. He has a war dog named Digger. That’s enough.