Cover image from Goodreads, I think. I have this same edition and actually the aspect ratio of the image is a little off — this image is a little squashed along the Y axis. The serpentman is actually more slender and tall on the cover, making the image more unsettling.
The book is a one-off adventure, as far as I know — the central character does not reappear in Brackett’s other books set on Mars, though the same world is explored in some of her other books under the name “Skaith”. As you may have guessed from the location on Mars, this book was written as an homage to the E.R. Burroughs “Carson of Venus” and “John Carter of Mars” books, and like them it is a very light but enjoyable adventure yarn, full of cliffhangers, occasional swashbuckling, and card-board thin characters (especially the protagonist). I think that for Burroughs, the undefined protagonists are meant to allow almost any reader to identify with them. Maybe Brackett intends the same thing. Either way, it is fun, with a lot of plot twists and action, very concisely written and never boring.
As a “planetary romance,” the story blurs the line between science fiction and fantasy. It takes place on Mars, involves time travel, and features aliens with psychic powers, so by modern standards it is more in the fantasy camp, but this is all presented as rational and scientific — the time travel and psychic phenomena is apparently just beyond the scientific understanding of the hero.
The villains of the book — serpent men and their human minions — remind me more of Howard than Burroughs. Brackett packs a lot into this very short novel, and it is worth a look.