Philip José Farmer, 1957, read as an ebook from manybooks.net
This is far from Farmer’s best, but it’s still a swashbuckling adventure. Reminiscent of the better Edgar Rice Burroughs adventures, it features technologically advanced humans trapped on a medieval backwater planet. As the story progresses, you find that maybe the world isn’t as primitive as our hero thinks.
It’s a fun ride, filled with what I’d call old-school wish-fulfillment sci-fi. Also notably, it features ships that sail through great grassy plains a good thirty or forty years before Dan Simmons’ Hyperion.