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.
The Ace Doubles in the database are listed below:
- The Big Time / The Mind Spider And Other Stories
- Contraband From Other Space / Reality Forbidden
- The Brains Of Earth / The Many Worlds Of Magnus Ridolph