Thuvia, Maid of Mars

Thuvia, Maid of Mars is a science fantasy novel by Edgar Rice Burroughs, the fourth of the Barsoom series. The principal characters are Carthoris (the son of John Carter of Mars) and Thuvia of Ptarth, each of whom appeared in the previous two novels.
While typical in many ways of Burrough's Barsoom novels, it also includes some inventive elements.
In this novel the focus shifts from John Carter, Warlord of Mars, and Dejah Thoris of Helium, 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 state/empires, and both Carthoris and Thuvia were secondary characters in the previous novel. Its plot devices are similar to the previous Martian novels, involving the kidnapping of a Martian princess. This time John Carter's son Carthoris is implicated. It does however have some inventive and original ideas, 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.