The Master Mind of Mars by Edgar Rice Burroughs

Ulysses Paxton, an Earthman like John Carter, is sent to Mars by astral projection. On Mars, Paxton is taken in by elderly mad scientist Ras Thavas, the "Master Mind" of the title, who educates him in the ways of Barsoom and bestows on him the Martian name Vad Varo. Ras has perfected techniques of transplanting brains, which he uses to provide rich elderly Martians with youthful new bodies for a profit. Distrustful of his fellow Martians, he trains Paxton as his assistant to perform the same operation on him. But Paxton has fallen in love with Valla Dia, one of Ras' young victims, whose body has been swapped for that of the hag Xaxa, the ruler of the city-state of Phundahl. He refuses to operate on Ras until his mentor promises to restore her to her rightful body. A quest for that body ensues, in which Paxton is aided by others of Ras' experimental victims.