Marion Zimmer Bradley 
The Door Through Space 
With linked Table of Contents

… across half a Galaxy, the Terran Empire maintains its sovereignty with the consent of the governed. It is a peaceful reign, held by compact and not by conquest. Again and again, when rebellion threatens the Terran Peace, the natives of the rebellious world have turned against their own people and sided with the men of Terra; not from fear, but from a sense of dedication. There has never been open war. The battle for these worlds is fought in the minds of a few men who stand between worlds; bound to one world by interest, loyalties, and allegiance; bound to the other by love. Such a world is Wolf. Such a man was Race Cargill of the Terran Secret Service.