I came across that term in an interview with the sound artist, Allen Mozek, and I instantly knew what he meant.
As a child, I remember that my mother would take me into this one, huge (it seemed to me) store. I think that it was a grocery store, but it might also have been something like a Target (though probably not a Target; maybe a Montgomery Ward). In that store was an enormous (again, it seemed so to me as a child) row of books and magazines. I loved best these hardback books that were mostly science fiction illustrations, like cover art for books and magazines. But that also had paperback fantasy and science fiction.
Recently, we visited my family in rural Arkansas. There was an old fashioned drug store that used to have a wire rack filled with cheap paperbacks.
Also, I can remember when supermarkets used to have books. I know that many still do, but back in the day, they were both cheaper and more varied.
I loved those supermarket fantasies. I wanted them all. I would read the backs of each one and pore over the covers with awe. Mozek is a little dismissive of them, but I am still engaged in an archaeology of those memories (hence, my love of Ace Doubles).