Science fiction adventure novel, expanded from a story which appeared in a pulp magazine in the 30's. First published in book form in 1960, this is the Airmont paperback, which remained in print from 1964 through 1980. There were several printings: the first so states on copyright page and has a cover price of 40 cents, the final has a cover price of 1.50.
Used availability for Fletcher Pratt's Invaders from Rigel