Thuvia, Maid of Mars

Shifting from the focus on John Carter, and Dejah Thoris (the protagonists of the first three books in the series) to their son, Carthoris, prince of Helium, and Thuvia, princess of Ptarth. Helium and Ptarth are both prominent Barsoomian city states. The novel introduces some inventive and original ideas for the time it was published, including an autopilot and collision detection device for Martian fliers, and the creation of the Lotharians, a race of ancient martians who have become adept at telepathic projection, able to create imaginary warriors that can kill, and sustain themselves through thought alone.