Ace published
science fiction,
mysteries, and
westerns, as well as books in none of those
genres. Collectors of its three main genres have found the Ace doubles an attractive set of books for the unusual appearance of the dos-à-dos format. Eight of the doubles, listed below, do not fit any one of the three genres. Each listing gives a publication date; in all cases that is year of publication by Ace, with the given catalog number, not the earliest publication date of the contents. 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==