Photographic reprint of two unknown editions, probably as they were reprinted (as two of the three novels) in the 1974 Ace paperback omnibus The Worlds of Poul Anderson. Omnibus edition of Question and Answer (variant of Planet of No Return), and World Without Stars. Question and Answer was originally serialized in Astounding Science Fiction June - July 1954. World Without Stars was originally serialized as The Ancient Gods in Analog Science Fiction/Science Fact June - July 1966. Both books had prior US paperback and US and UK hard cover publications.
Used availability for Poul Anderson's Two Worlds
September 1978 : USA Hardback