The Chessmen of Mars (1922)
Fifth of the eleven books of the Barsoom series (the last of the five novels now in public domain). John Carter is not a main character of this one. For this novel Burroughs created an interesting new race, the Kaldanes. They sacrificed their bodies to become pure brains and have bred a symbiotic race of headless human-like creatures called Rykors, which they can attach themselves to and ride like a horse. (more on wikipedia) Burroughs also created Jetan,the Martian version of chess.