Beyond the Farthest Star by Edgar Rice Burroughs was published in 1964 by his estate, fourteen years after his death. It consists of two related novellas -- the first, "Adventure on Poloda" had been published in a magazine in 1942, but while the second, "Tangor Returns," was probably written at about the same time, it wasn't published until it was found in Burroughs papers. It seems likely that Burroughs intended this story to take off into a series, and it has the same hallmarks as the John Carter or Pellucidar novels (through extraordinary circumstances, an all-American guy finds himself in a strange world and is initially threatened but ends up taking care of business).
In Beyond the Farthest Star, our American hero is shot down over Germany during WWII, feels himself dying, and then wakes up on another planet, completely naked. He stands up and sees a beautiful woman in a crazy glittery jumpsuit. She screams and runs and then five men run out from under a small hill and take him captive. He has somehow been transported to Poloda, a planet so far away from Earth that it really is "beyond the farthest star," and a planet that has been at war for over 100 years.
Our hero had the good luck to magically appear in the country of Unis, the good guys in the war, and the Unisians give him the name of Tangor. The Unisians would love to have peace, but they are stuck defending themselves against the ever-more-aggressive Kapars who have subjugated the rest of the planet. Over the last century of warfare, the Unisians have dug out underground cities with buildings that can be raised up to the surface and then lowered when there is a raid by the Kapars. Lucky for Unis, Tangor was a good soldier and excellent pilot back on Earth and he quickly joins the forces defending Unis from the air, and proves himself a mega-hero.
In the second novella, Tangor is approached by a Unisian woman who sympathizes with the Kapars and wants to defect to their country and double-cross the Unisians. Tangor tells his superiors, who have him go along with her plan and work as a double-agent in Kapar. Kapar is an extremely repressive society, ruled by a secret police and torn apart by fear with neighbors and family informing on one another. Naturally, Tangor gets into some tough spots in Kapar, but ultimately gets the job done.
These aren't the most polished Burroughs stories, but they are an interesting product of the war years and come off as more political than much of his other work. The anti-war sentiments and the theme of fighting against a Stalinesque dictatorship are hard to miss. You should definitely read these if you are a Burroughs completest, or if you just like old science fiction novels with naked men on the cover.