ace's second title was a western (also "tête-bêche"): william colt macdonald's "bad man's return", bound with j. edward leithead's "bloody hoofs". mysteries and westerns alternated regularly for the first thirty titles, with a few books not in either genre, such as p. g. wodehouse's "quick service", bound with his "the code of the woosters". in 1953, a.e. van vogt's "the world of null-a", bound with his "the universe maker", appeared; this was ace's first foray into science fiction. (earlier in 1953, ace had released theodore s. drachman's "cry plague!", with a plot that could be regarded as sf, but the book it was bound with—leslie edgley's "the judas goat"—was not sf.) another sf double followed later in 1953, and sf quickly became an important part of ace's business. by 1955, the company released more sf books each year than in either of the other two genres, and from 1961 onward, sf titles outnumbered mysteries and westerns combined. ace also published some juvenile delinquent novels in the 1950s. collectors want these books, such as d-343, "the young wolves" by edward de roo and d-378, "out for kicks" by wilene shaw.