Back in the late 1960's, I was a sci-fi nerd who read novels starting when I was around six years old. Turns out I was an Asperger, but this was before that form of autism was understood or even "named" as a discovery in 1975. Nobody knew what was wrong with my brain but I read a lot, to the point where I was reading college astronomy and math textbooks when I was very young.
My favorite books were the old Ace Double books. As noted by Lord Brawl, they were two novels (around 40K words is "officially" a novel) stuck inside one bound book. When you were done with the first one, you flipped it over (bottom edge up) to read the second story.
Many of the novels were butchered versions of longer ones, and some were produced by prolific authors specifically for the Ace Double lines.
I remember there was one particular one that had a giant buzz saw on the cover, and I really enjoyed reading it. When I was an old phart, I wanted a copy for my collection. After a lot of digging at used bookstores and in the dealer rooms of conventions, I found one. "Battle on Venus" by William F. Temple, and the other story was "The Silent Invaders" by Robert Silverberg. I recall liking the first one more, but after re-reading them decades later, I can say that the venerable Silverberg was the superior writer. Both were excellent pulp science fiction stories for a young mind. Back in the day, I could buy a comic book for ten cents or, if I saved up four dimes, I could get two novels for fourty cents, brand new off of the spinner rack at the local convenience store.
The paper quality was, well, pulp. All of my Ace Double copies are heavily yellowed with age, but the spines seem to be holding up to this day. I have a large collection on several bookshelves, all of them well-worn reading copies.