In the 1950s, Ace Books began publishing Ace Doubles, which were two separate novels combined in a single volume. For example, a reader could read “Secret Agent of Terra” which ends at about the middle of the book. The reader could then close the book; do a head-to-toe flip of the volume; and there would be the cover of the second book, The Rim of Space.
The reader is happy because they get a twofer—two separate novels for the price of one (which was $0.40 back then). The two-novels-in-one book concept clearly provided great value to the avid reader.