List of Ace double titles
American company Ace Books began publishing genre fiction starting in 1952. Initially these were mostly in the attractive tête-bêche format, but they also published a few single volumes, in the early years, and that number grew until the doubles stopped appearing in about 1978. The tête-bêche format was discarded in 1973, but future double novels were continued for a while. Between 1952 and 1968, the books had a letter-series identifier; after that date they were given five-digit numeric serial numbers. The list given here covers every Ace Double published between 1952  and 1978, for all genres. It 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. 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. 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 includes an indication of the genre of the works, in italics after the serial number. Abbreviations used are "SF" for science fiction titles; "MY" for mystery titles, and "WE" for westerns. In addition, "NA" is used to mean "not applicable", when one or both of the books is not in one of these three genres; and "UN", when the genre of the books is not known. The list also 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 thought to be complete, but there may be minor omissions among the later lists. 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. Jokes. The double format inspired contests for ironic or satirical combinations of titles that might appear, as in "No Blade of Grass"/"The Sheep Look Up" References. The following references have not been seen: List of Ace double