Marion Zimmer Bradley - Door Through Space

At one time Race Cargill had been the best Terran Intelligence agent on the complex and mysterious planet of Wolf. He had repeatedly imperiled his life amongst the half-human and non-human creatures of the sullen world. And he had repeatedly accomplished the fantastic missions until his name was emblazoned with glory.

But that had all seemingly ended. For six long years he'd sat behind a boring desk inside the fenced-in Terran Headquarters, cut off there ever since he and a rival had scarred and ripped each other in blood-feud.

But when THE DOOR THROUGH SPACE swung suddenly open, the feud was on again—and with it a plot designed to check and destroy the Terran Empire. (original book blurb)