Rogue Soldier: One Man's War (Unabridged‪)‬

He called himself Staff Sergeant Keith, but word around C Troop had it that the spooky guy in tiger suit fatigues wasn't an enlisted man, maybe not even army. Some thought he was from CIA. But the air cavalry troops were told that he had their commander's blessing, so they took him along. 

C Troop was engaged in its toughest fight of the war, flying South Vietnamese soldiers and supplies across the border into Laos in an audacious attempt, late in the war, to turn the tide of battle, cut American losses, and shorten the war. The North Vietnamese Army waited in hiding, picking off helicopters, and, when the time was right, battering South Vietnamese ground forces with tanks, heavy artillery, and everything they had.

Staff Sergeant Keith was no ordinary soldier. Keith had fought with the Special Forces, spoke Mandarin Chinese, and was trained to intercept signals from enemy radios and phone lines, analyze them, and send what he learned through highly classified channels to the National Security Agency and other intelligence outfits bent on outsmarting the enemy. 

He decided on his own he could be more useful helping helicopter crews spot their targets before the targets shot them down. He also believed he had a gift for doing that. Keith flew numerous missions, unauthorized by and unknown to his own commanding officer. His luck ran out, however, and an enemy machine gun ended the war for Staff Sergeant Keith. It was the beginning of a painful, lifelong struggle.