1956 "First Edition" stated on copyright page; The John C. Winston Company publishers, Philadelphia; hardbound in green boards with illustrated endpapers; quite good condition with unmarked pages, except for inside front cover writing (see pic); no dust jacket.
 
Description -
A clandestine band of humans work behind the scenes to prevent a war between Earth and the octopus-like inhabitants of Poseida. Part of the classic Winston science fiction series for young readers published between 1952 and 1961.