The Asset

In this novel by Shane Kuhn, our protagonist is a man who lives an existence driven by guilt  because he ignored his sister's request to fly home with her and the plane was one of those that crashed into the twin towers on 9/11. Since then, Kennedy has studied behavioral indicators of terrorists, notably with the Israelis, and has become the "go-to" trainer for TSA agents in the United States and abroad. He practically lives on airplanes, and when he is "home" in LA, he lives in a hotel. He is approached by the CIA to help determine what sort of master plan is going to be implemented by a nearly invisible terrorist who manipulates world events from behind the scenes to keep himself funded. Kennedy initially agrees to help, then quits when two sleeper terrorists working as TSA agents are killed. When he decides he wants back in, he has to prove his value by tracking down the terrorist on his own, with a little help from his sister's former best friend, pop star Love.
This is a very twisty plot and you trust all the wrong people along the way, but it draws you in and along until the very end when the resolution is just so unbelievable. Kennedy single-handedly disarms a nuclear weapon set to go off in Atlanta airport and ultimately becomes a member of the CIA, along with his new love, Love.