The Goddess of Ganymede by Michael D. Resnick (1967)
For his very first book in the fantasy field, Mike Resnick channeled his inner Edgar Rice Burroughs.
(Let me back up just a little bit. Resnick actually published a previous story, "The Forgotten Seas of Mars," that was a sequel to Burroughs' Llana of Gathol. This 26,000 word story was published in a limited edition by EBR-dom with the full approval of the Burroughs estate. But that was a story, . not an novel, and it was published by the amateur press, not a professional -- albeit fledgling -- publisher like Donald M. Grant. Resnick, of course, was not new to the writing field. He had written over 200 soft core novels under pseudonyms and had been a columnist and a magazine and newspaper editor. Resnick had the chops and The Goddess of Ganymede was his first major entry into a long, distinguished, and on-going career in speculative fiction.)
So, Ganymede. Not Mars (Barsoom) or Venus (Amtor), but the largest of Jupiter's (and the solar system's) moons. Because long before man made his first suborbital space flight, the United States government was operating two secret space programs in addition to the one the public knew about. Of the two secrets programs, one -- Project Jupiter -- looked the most promising. Unfortunately, the first two attempts to reach Jupiter failed. In their wisdom, the powers that be decided that their third try might have a better chance -- or, at least, a less complicated result if it failed -- with a crew of one: an experienced pilot not associated with the government. It happened that such a pilot was the experienced adventurer Adam Thane.
So Adam Thane blasts off for Jupiter and Jupiter's gravity captures his ship, pulling Adam to a certain death on the gas giant. Luckily Adam's course has also brought him close to Ganymede and its weaker gravitational pull. Adam uses most of his fuel trying to push his ship away from Jupiter and to its moon. His gamble works and Adam finds himself on a stranded on a strange planet.
Ganymede, or Kobar, as its inhabitants call it in their universal language, is essentially a world of city-states. One hemisphere of the moon had been devastated in an ages-old war, the radiation from its super-weapons has spawned mutated races. On the other hemisphere live a human-like race, their skins a golden color. Most technology has been abandoned after that war thousands of years ago; preferred weapons now are swords, knives, spears, and maces. Each kingdom (city-state) has a Togron (or king). The kings sons carry the title Gron. A Gron may be the Togron's biological son, or someone adopted by the Togron. Kobar is under the thumb of Malthor, the "City of the Gods." The gods -- cruel, callous, and vengeful -- number about a hundred with Tarafolga as the main god. The "gods" are actually men made immortal by scientific means and who have created a religion to solidify their powers and exercise their depravities.
That's the basic set-up. Adam Thane, like John Carter has extraordinary powers due to Kobar's lower gravity. Taken by the flying beings from the Kingdom of Kroth, Adam soon is named a Gron by the kingdom's leader, Balor. A war is brewing between Kroth and the human kingdom of Rambus. Adam leads a regiment of Kroths and soon discovers that the war is being manufactured by Malthor with the help of a treacherous Rambian. Adam soon meets and falls in love with Delisse, the daughter of the Togron of Rambus. Another pawn of war, Delisse is captured and taken to Malthor to be wed to Tarafolga, She is named a princess, soon to become a queen of Kobar.
There are battles and sword fights, captures and escapes, strange beasts and beastly men, huge palaces and deadly pits, evil villains and a noble hero -- all the trappings of a great Burroughs novel. And a happy ending? Not quite, but a satisfying enough ending to set up a sequel, Pursuit on Ganymede, which was published the following year by Paperback Library.
A good book and a good beginning to a storied career. The thirteen-year-old in me says thanks to Resnick for this one and all that followed.