Marion Zimmer Bradley 
The Door Through Space [EPUB ebook] 

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!