Just finished Black Amazon of Mars by the late great Leigh Brackett.
Nothing groundbreaking or thought provoking, but really fun and pulpy all the way through. The oldest piece of fiction I’ve read (save for snippets of The Odyssey and Inferno) is The Martian Chronicles by Ray Bradbury, so perhaps I need more experience to gauge this correctly, but Black Amazon feels way ahead of its time.
I’d recommend checking out A Princess of Mars by Edgar Rice Burroughs. One of the great scifi stories that influenced all scifi that came after it. Including both George Lucas and Gene Roddenberry.
And if you’d like to check out more classic pulp action, I recommend the Doc Savage stories. Starting with The Man of Bronze by Lester Dent.