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.
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 include SF (science fiction), MY (mysteries), WE westerns, and NF (nonfiction).
The genre of certain books can be deduced by the title or by reading about the author: TH for thriller, PI for pirates, HU for humor, WA for war, HO for horror, RO for romance, HF for historical fiction, and JD for juvenile delinquents.