Ace Double F-209 (1963): containing Let the Spacemen Beware! by Poul Anderson, bound with The Wizard of Starship Poseidon by Kenneth Bulmer. This is the first American paperback appearance for either novel.

Let the Spacemen Beware! (cover by Ed Emshwiller, 98 pages) is expanded from "A Twelvemonth and a Day" in Fantastic Universe in January 1960, and was retitled The Night Face in the late Seventies.

The spaceship Quetzal had a mixed crew and simple mission: negotiate a site for a refueling station on the planet Gwydion. The alliance of republican Nuevamericans led by Miguel Tolteca with the aristocratic, military-minded Lochlanna, headed by Raven, should be able to deal with the amiable inhabitants of Gwydion - who were unfamiliar with the concepts of thievery, murder and warfare, and had only wind and water power. But the Gwydions had a secret: a mysterious cycle and a day of danger, when the Night Faces ruled.

(bound with) The Wizard of Starship Poseidon (cover by Jack Gaughan, 124 pages) is original (no previous magazine version.) Professor Cheslin Randolph was barely 5 feet tall, and a genius in the field in biology. When he finds that 10 years of research and preparation are about to be wasted because of fickle University funding, he turns to crime so that his life-work can be realized. He teams up with his nephew Terry Mallow, a cashiered Navy officer, with a plan to rob a spaceship in mid-flight - the starship Poseidon.