Alcatraz of the Starways by Albert dePina

Venus was a world enslaved. And then, like an avenging angel, fanning the flames of raging revolt, came a warrior-princess in whose mind lay dread knowledge—the knowledge of a weapon so terrible it had been used but once in the history of the universe.   A thrilling short novel.

The Sky is Falling by Lester Del Rey

Computer engineer Dave Hanson is transported from Earth to a world where the rules of science and physics are replaced by the rules of magic. Hanson was summoned to help save the world's sky from collapsing. The sky is a literal dome that arches above the world, with the sun and stars not massive celestial bodies trillions of miles away, but smaller lights that move about within the dome of the sky.
Hanson must find a way to reconcile his scientific knowledge with the magic of his new world and thus prevent the sky from falling.