Philip Jose Farmer 
Cache from Outer Space [EPUB ebook] 

Benoni Rider set out across the unexplored desert of a future America to prove himself a man and find a new land for his people. The task at first seemed merely exceedingly hard – and then it began to seem entirely impossible. Because all he had to do was join a barbaric army, become a bodyguard for a queen, act as another nation’s emissary to his own, lead an army into battle against the wild men of the north, and manage somehow to get back to Fiiniks with the secret of the CACHE FROM OUTER SPACE. That last was the secret that, if learned, could enable any of the barbarian nations of that devastated future to control the rest of the world . . . or annihilate it all over again!