Basically, this is a bunch of short stories by this husband and wife team of science fiction writers packaged as an Ace Double.
Ace Doubles were usually two novellas, sometimes by two different authors, sometimes not, packaged in one paperback, one reversed from the other. They were cheap, the price of a single inexpensive paperback, and commonly found in the carousel racks of pharmacies and the like. For kids like me with a quarter to spend, they were a godsend, favored not only for their prices but also because the two titles doubled the chances of a good read. They also, from a teacher or parent's perspective, doubled the chance that the kid would get into a particular author and go on reading.
In seminary one of my better friends was John Calvin Batchelor, now an established author. He had cut his authorial teeth on Ace Doubles under a variety of pseudonymns--another common practice of Ace being to combine a name author with an unknown, Kilgore Trout kind of writer. By the time we were in seminary together he was producing even more disreputable literature, also under pseudonymns, while contributing to a variety of more-or-less countercultural magazines and newspapers. Only after graduation did I find him in, of all places, The New York Times' book reviews--finally under his own name.
So, on behalf of readers and writers--thanks, Ace!
PS See the Wikipedia article on Ace Doubles cited.