Ace Double D-223 (1957): This Fortress World by James Gunn, bound with The 13th Immortal by Robert Silverberg.

This Fortress World (abridged) - cover by Ed Emshwiller; 190 pages. The hardcover from Gnome Press came out in 1955. The galaxy itself had suddenly become too small to hide William Dane. For he had stumbled on the secret that every power-mad faction of every planet would kill and torture to obtain. So he learned the ways of the hunted - how to melt into shadow, how to don other men's identities like masquerade costumes. And then one day Dane could run no more. He had to face his final pursuers - and possibly death itself. But then he knew that if somehow he could learn in time how to use his secret he could save himself… and with him all mankind… from annihilation.

(Bound with) The 13th Immortal - cover by Ed Valigursky; 129 pages. This appears to be original - with no earlier magazine source. Dale Kelsey could not even remember who his father was - his past was an utter blank. But he knew one thing - the answer to his life's riddle lay in Antarctica, the once frozen continent, now an earthly paradise surrounded by an impenetrable barrier. But how to get there? The only means of transportation were the spindly six-legged mutant horses. And it was suicide for Kelsey to travel on the American continents. Two immortal dictators had set king-size rewards for his capture - dead or alive. But somewhere in the 2 continents there was someone who would help him, someone he had to find. The future of the world depended on his success.