Ace Books began publishing genre fiction in 1952. Initially these were mostly in the tête-bêche (double) format, but they also published a few single volumes in the early years. The tête-bêche format was discarded in 1973, with later doubles being published adjacent to each other. Between 1952 and 1968, the books had a letter-series identifier; and after that date they were given five-digit numeric serial numbers. Ace stopped publishing its doubles series sometime around 1978.
Number in series: 72401