Ace Books published 221 science fiction Ace doubles between 1952 and 1973 in tête-bêche format, and a further 40 between 1974 and 1978 in a more traditional format in which the two books are both the same way up. Genres and collectability Ace published science fiction, mysteries, and westerns, as well as books not in any of these genres. Collectors of these genres have found the Ace doubles an attractive set of books to collect, because of the unusual appearance of the tête-bêche format. This is particularly true for the science fiction books, for which several bibliographic references have been written (see the References section). The format inspired a further series of sf doubles published by Tor Books between 1988 and 1991, the Tor Double Novels. Because the tête-bêche format is part of the attraction for collectors, some do not regard as true Ace Doubles those books published between 1974 and 1978, which contain two works by one or two authors bound traditionally rather than back-to-back and upside-down (tête-bêche format). All volumes containing two SF works, in the entire 1952 to 1978 series, are listed below, with the late change in format noted. Each listing gives a publication date; in all cases, that is year of publication by Ace, with the given catalog number, not earliest publication date of the contents. The list is complete for books containing two science fiction titles. Number D-13 in the official series, listed in Miscellaneous Ace Doubles, contains one novel, Cry Plague! by Theodore S. Drachman, which can be regarded as science fiction. Therefore some science fiction collectors treat D-13 the first science fiction Ace Double, although the novel on the other side of the book is a mystery, The Judas Goat by Leslie Edgley. For more information about the history of these titles, see Ace Books, which includes a discussion of the serial numbering conventions used and an explanation of the letter-code system. D Series D-031 SF A. E. van Vogt The World of Null-A / The Universe Maker (1953) D-036 SF Robert E. Howard Conan the Conqueror / Leigh Brackett The Sword of Rhiannon (1953) D-044 SF Donald A. Wollheim (ed.) The Ultimate Invader and Other Science-Fiction / Eric Frank Russell Sentinels of Space (1954) D-053 SF Murray Leinster Gateway to Elsewhere / A. E. van Vogt The Weapon Shops of Isher (1954) D-061 SF L. Sprague de Camp Cosmic Manhunt /