Published by Ace Books in the 50s and 60s, the Ace Doubles were two short novels bound back to back (so that when you got to the middle, you turned the book over and upside down to read the second book). Each novella¹ got its own cover.
Most of the Doubles were science fiction, mysteries, and westerns.
The science fiction series published novellas by some of SF's biggest names including Marion Zimmer Bradley, Eric Frank Russell, A. E. van Vogt, L. Sprague de Camp, Clifford Simak, Gordon R. Dickson, Harlan Ellison, Andre Norton, Samuel Delany and more.
Now a collector's item, collectors will pay anywhere from $2 to $60 for a copy of the rarer books.
1. Back in the day the publisher held considerable power, many longer works were eviscerated to fit the Double format.