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, titles published between 1974 and 1978, which contained two titles by one or two authors but which are not tête-bêche are not regarded by some collectors as true Ace Doubles. The distinction is up to each collector; the books are included in the list given below, with the difference in format noted.
The list given here gives a date of publication; in all cases this refers to the date of publication by Ace, and not the date of original publication of the novels. The list is complete for science fiction titles. However, D-13, listed in Miscellaneous Ace Doubles, contains one novel, Cry Plague! by Theodore S. Drachman, which can be regarded as sf, and some science fiction collectors treat this as the first sf Ace Double, even though the novel on the other side is not sf in any way.
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.
Read more about this topic: List Of Ace SF Double Titles