Ace Double D-223 (35 cent cover price) 1957 paperback. Condition is Good+ to VG-: tight and square; spine is slightly raised down middle and barely sagged on either side (I'm guessing the glue has collapsed or is depleted?) - along with wear and edgewear; light to moderate overall wear including lines next to spine and edgewear (see scan.) 2 small holes in upper middle of Gunn side; some tape residue/traces on insides of covers. On the Silverberg side that residue might indicate where a plastic covering to the book was anchored, but I have no idea what the pattern on inside of Gunn cover means (see scan #3.) Age tanning is mild to moderate and uniform (consistent with its vintage.) Stamps on first page & page 85 (each side) - from the reserves of a defunct SF library; no other marks or writing. A lot of small faults - but Doubles over 50 years old are not getting any easier to find.

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.