Windmills of the Gods by Sheldon, Sidney

She’s on the glinting edge of East-West confrontation, a beautiful and accomplished scholar who has suddenly become our newest ambassador to an Iron Curtain country, a woman who is about to dramatically change the course of world events — if she lives. For Mary Ashley has been marked for death by the world’s most proficient and mysterious assassin, and plunged into a nightmare of espionage, kidnapping, and terror. Here, only two people — both powerfully attractive and ultimately enigmatic men — can offer her help. And one of them wants to kill her.