Keith Laumer 
Retief: Diplomat-at-Arms [EPUB ebook] 

Retief had just one job on Northroyal—to save the galaxy from madness and war. So with a frayed cloak and an old horse and a packet in his saddlebags—not to mention blood, guts, and brains he set out.
Keith Laumer was a Hugo and Nebula award nominee! Before becoming a science fiction writer Laumer was an officer in the United States Air Force and a diplomat in the Foreign Service, adding a note of realism to many of his stories. One of science fiction’s true luminaries.