I've read three Captain Future novels and I am surprised at how enjoyable they can be. Originally published in 1945, "Outlaw World" pretty much defines the concept of science-fantasy.
Curtis Newton, better known as Captain Future, and his comrades blaze about the solar system after mysterious radium pirates who are, somehow, managing to intercept almost all shipments of the vital fissionable material. What are the pirates doing with all that radium? How are they locating the shipments despite all attempts at secrecy?
The thing which elevates this particular adventure is the wealth of wonderful environments:
The gigantic flower-forest of the asteroid Leda.
The Pirate asteroid known as Iskar.
The Asteroid Zuun where giant cave-apes live.
Vulcan, the hollow world just outside the Sun's corona.
Lovely Frazetta cover on the Popular Library paperback, although you can bet Captain Future wasn't running around naked like the guy in the painting, that sorta thing is more in John Carter's line.