Naked to the Stars

Cal Truent was a soldier, and a good one, right up to the moment he took a hit during action on the third planet from Arcturus.

Cal woke up in the hospital, expecting a short vacation and maybe a medal and than back to work. But there was a problem: the doctors found a sixteen-hour hole in Cal's memory. No one knows what it is that Cal has forgotten, but his superiors can't take the chance that it might be something deadly to his fellow soldiers – and to Earth

Cal is barred from combat duty, "promoted" to a non-combatant Contacts Officer, one of a breed of diplomats he has always despised. But he has sworn to himself to clear his name; somehow Cal means to seek out whatever it is that his mind is resisting, to face it squarely, and to overcome it – as becomes a soldier.