From epubbooks.com:

The shimmering, cloud-covered planet of Venus conceals a wondrous secret: the strikingly beautiful yet deadly world of Amtor. In Amtor, cities of immortal beings flourish in giant trees reaching thousands of feet into the sky; ferocious beasts stalk the wilderness below; rare flashes of sunlight precipitate devastating storms; and the inhabitants believe their world is saucer-shaped with a fiery center and an icy rim. Stranded on Amtor after his spaceship crashes, astronaut Carson Napier is swept into a world where revolution is ripe, the love of a princess carries a dear price, and death can come as easily from the blade of a sword as from the ray of a futuristic gun.

From epubbooks.com:

Second in the Venus series. Carson Napier begins this episode in the Room of the Seven Doors. He can leave any time he wants, but six of the seven doors lead to hideous deaths; only one is the door of life. After navigating his way out of this logic puzzle, Carson continues his quest to rescue the planet’s fairest princess. He pursues this with singlemindedness, even though more terrible dagners lie ahead; even though the princess wishes neither his help or his affection; even though her people will execute him if he enters their country! Such is the honor of an Earthman’s pledge.

From epubbooks.com:

Carson Napier is the first Earthman to visit Venus, and he has to keep alert every moment of his stay in that world of mist and mystery. For its territory is unmapped, its inhabitants strange and he has undertaken to restore a native princess to her lost homeland. In terrible oceans where dreaded sea monsters dwell, in deep forests where fear haunts every branch, and behind the walls of eerie cities where power-mad chieftains plot uncanny schemes, Carson of Venus faces damagers that no man ever faced before.

From epubbooks.com:

Earthman Carson Napier had found his share of adventure on the cloud-shrouded planet of Venus. He had rescued his beloved Princess Duare from one peril after another. But when Carson finally restored Duare to her home in the lofty kingdom of Vepaja, she was sentenced to death for daring to love a lesser mortal! Once again, Carson rescued the Princess, snatching her from her own father’s palace in an airship of his own devising.

The two outlaws searched the misty skies of Aptor for a refuge, but found only trouble. From the country of the amphibian people to Voo-ad, the city of human amoebae, they faced dangers beyond imagining.

From epubbooks.com:

The final adventure of Carson Napier among the exotic peoples and beasts of Amtor is the sequel to his fabulous four Venus novels, it is an adventure not to be missed as Napier encounters a new kind of science and a new master of alien deviltry.