Notes
The 13th Immortal, a standalone novel by Robert Silverberg.

This volume is a "dos-à-dos" omnibus in the Ace Double series. It also includes This Fortress World by James E. Gunn.

The image opposite and the blurb below are from the 2004 Cosmos Books paperback edition, ISBN 0843959517. Cover design by J.T. Lindroos.

Publisher's Blurb – Lower Cover
FORBIDDEN CONTINENT

Who was your father?" the mutant asked Dale Kesley. And try as he might, Kesley could not remember; 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 imprenetrable barrier.

But how to get there? The only means of transportation were the spindly six-legged mutant horses. And it was suicide for Kesley to travel on the American continents. Two immortal dictators had set king-size rewards for his capture – dead or alive! But somewhere in the two continents there was someone who would help him, someone he had to find. The future of the world depended on his success.