The Door Through Space

... 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.

Marion Eleanor Zimmer Bradley (1930 - 1999) was an American author of fantasy, historical fantasy, science fiction, and science fantasy novels, and is best known for the Arthurian fiction novel The Mists of Avalon, and the Darkover series. While some critics have noted a feminist perspective in her writing, her popularity has been posthumously marred by multiple accusations against her of child sexual abuse and rape by two of her children, Mark and Moira Greyland, among many others. Zimmer Bradley's first child, David R. Bradley, and her brother, Paul Edwin Zimmer, also became published science fiction and fantasy authors.