Kenneth Bulmer 
Earth’s Long Shadow [EPUB ebook] 

Visa for an enigma.When John Carter came to the Horakah Cluster, it was in the guise of an interstellar salesman. If anyone there suspected he was more than that, it would mean his instant execution.But Carter’s unusual personality made it possible for him to put over the deception and even gain a visa to the forbidden central planet, an arsenal of space war factories. Of course, had to make some special deals to do it, and those proved his undoing.For he found himself caught between two menaces: the tyrannical militaristic moguls and a fantastically greater threat fro beyond the ends of space.