The Master Mind of Mars (Barsoom #6)

The Master Mind of Mars is a science fantasy novel by American writer Edgar Rice Burroughs, the sixth of his Barsoom series. Burroughs' working titles for the novel were A Weird Adventure on Mars and Vad Varo of Barsoom. It was first published in the magazine Amazing Stories Annual vol. 1, on July 15, 1927. The first book edition was published by A. C. McClurg in March, 1928.
Burroughs had been unable to place the novel in his standard, higher-paying markets like the Munsey magazines and the Street & Smith line. Some critics have speculated the publishers were put off by its satirical treatment of religious fundamentalists. He eventually sold it to publisher Hugo Gernsback for $1,250: only a third of the rate paid by magazines like Argosy All-Story, where the previous book in the series had first appeared. Gernsback chose the novel's final title and made it the cover feature in his newest magazine.

In this novel Burroughs shifts the focus of the series for the second time, the first having been from early protagonists John Carter and Dejah Thoris to their children after the third book. Now he moves to a completely unrelated hero, Ulysses Paxton, an Earthman like Carter who like him is sent to Mars by looking at the red planet in the sky.
Original 1927 magazine publication
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, Jeddara (empress) 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, and in the end he attains the hand of his Valla Dia, who in a happy plot twist turns out to be a princess. (wikipedia.org)