The first of the Mars books.
The opening's a bit marred by the conventions of the day -- explaining how Burroughs got his hands on the story. And the transition to Mars is not smooth on either end. (Personally, I would have given him some sort of Martian connections and had him transported by their tech, but Burroughs did not have the advantage of working in a field with many developed tropes.)
Having been improbably transported to Mars, John Carter finds himself immensely strong, having to remaster walking rather than bounding about like a grasshopper. He also falls in with the ferocious green men of Mars, having landed near their incubation dome. As their prisoner, he learns an immense amount of their world, and fights a couple of time, too, thus winning rank in their eyes.
But when the green men attack an airship and capture the title princess, Dejah Thoris, the action starts to pick up. It involves a station where the Martian atmosphere is maintained, murals of a previous prosperous civilization, the law that a woman can not marry the man who killed her betrothed, the advantages of kindness in taming beasts, gladiator games, winning a position by saving a high-ranking man's life, a secret romance among the green Martians who mate, usually, strictly on eugenic grounds, savage white apes, and much more.