Marion Zimmer Bradley 
The Door Through Space 

At one time Race Cargill had been the best Terran Intelligence agent on the planet of Wolf. He had repeatedly imperiled his life amongst the half-human and non-human creatures. And he had repeatedly accomplished the fantastic missions until his name was emblazoned with glory. But that had all seemingly ended. Now for years he sits behind a boring desk inside the fenced-in Terran Headquarters. But when THE DOOR THROUGH SPACE swings suddenly open, a riproaring space feud adventure begins – and with it a plot designed to check and destroy the Terran Empire!