Edmond Hamilton 
Closed Worlds 

When Morgan Chane and his comrades of John Dilullo’s interstellar mercenaries invaded the Close World of Arkuu in search of a lost Terran expedition, they found a planet of strange menace. Incredibly powerful monsters prowled though Arkuu’s dense jungles, and the ghosts of the planet’s past haunted its ancient deserted cities. The Arkuuns themselves fought grimly to drive the Terrans away. But at last Chane discovered the Free-Faring, the terrible alien secret of Arkuu..and suddenly he knew why no Terran had left the Closed Worlds alive.