Jaffery, Sheldon Double Trouble: A Bibliographic Chronicle of Ace Mystery Doubles, Starmont Popular Culture Series no. 11, Borgo Press, 1987. . Jaffery, Sheldon Double Futures: An Annotated Bibliography of the Ace Science Fiction Doubles, Borgo Press, 1999. . Peters, Harold R. Science Fiction, Fantasy & Horror in the Ace Letter-Series Editions: A Collector's Notebook, Silver Sun Press, 1996. SF Double Titles Ace SF Double Titles Ace SF Double Titles