Silverberg used this title twice, in 1959 and 1961, for what are apparently distinct works.
Other resources
(None on file)
Acknowledgements: Robert Silverberg for cause, support, and cooperation; Ken Seamon for graphics; Rodney Walters for books and info; Alvaro Zinos-Amaro for comments and content; and all the fellow fans who have sent encouragement, information, and translations.