Tharkol, Lord of the Unknown by Edmond Hamilton (1950)
Edmond Hamilton, pulp master extraordinaire, burst on the scientifiction (as opposed to science fiction) scene in 1926 for a career that lasted just over half a century. His early stories gave him the nickname of "Worldwrecker" Hamilton because of the way he blithely destroyed planets with his purple prose. His penchant for action, adventure, and super-science eventually brought him to the creation of Captain Future, one of the icons of the pulp SF field. There was another to Hamilton, though, a thoughtful and literate side which appeared in his later career but had always -- as also with his wife, Leigh Brackett -- been lurking behind much of his fast-moving, pulse-pounding early fiction.
Tharkol, Lord of the Unknown, however, is pure escapist fiction. First published in the third issue of Startling Stories (May 1939) under the title "The Prisoner of Mars," the tale was published in paperback form by Consul Books in Australia; as far as I can tell, this was the only book publication of the novel. Also, as far as I can tell, the book's title is virtually meaningless. The story is not about Tharkol and nowhere was he called the Lord of the Unknown, but it's an interesting title, don't you think?
Thirty years previous, a naked and wounded man was found wandering in a Canadian forest. The man had no memory of his past and, as it turned out, would never regain it. The man was taught English, given the name John Crain, and eventually moved to the United States, married, and had a son, Philip Crain. As the novel opens, Philip and a few friends are listening to a Mercury-Theater-War-of-the-Worlds type radio program, when Philip suddenly has a panic attack and vague...memories(?) of an alien invasion. Philip's father, dead these past five years, turns out to have been an alien -- Tharkol, King of Mars, sent to Earth on a doomed mission to save his dying planet by robbing Earth of its oceans. Martians, by the way, have a mild sort of racial memory that is passed on genetically, thus Philip's vague memories.
Philip and his friends, thinking John Crain might have been a downed airplane pilot, journey to the Canadian wilds in an effort to find the remains of his father's ship. What they find is a type of spaceship and a matter transmitter which brings Philip to Mars. With the knowledge that the matter transmitter works, the long-delayed plan to rob Earth of its water goes ahead. Because Mars is losing its water, the population has been reduced to only a million persons residing in the five remaining Martian cities, while millions more Martian have been placed into a mindless stasis to ration the planets dwindling resource.
Eager for the planet to be revived are the revered scientist, Dandor, and the current king of Mars, Lanu, who happens to be Crain's half brother and identical look-alike...and the plot begins to take a Graustarkian turn. Naturally, there are evil people plotting to take the throne.
The Martians had once developed amazing robots, but the fear that the robots might rebel against them convinced the Martians to destroy all of the robots except for one -- Kro, a giant robot given to Dandor out of respect to Dandor's contributions to science. One robot that was assumed destroyed was The Brain, a super-thinking machine whose intellect caused it to disdain the Martian Race. (The Brain is a robot because the concept of a supercomputer was not around in 1939.) The Brain, of course, was not destroyed but is now working with the bad guys.
Tharkol, Lord of the Unknown is rip-snorting planetary adventure, filled with marvelous inventions, ray guns, evil plots, danger, robots, giant mechanical worms, insurrection, mistaken motives, impending doom, treason, forbidden love, a race against time, and much more. At the center is conflicted Philip Crain, half-Martian and half-Earthling. No matter which side he chooses, the other side is doomed. Or is it? Can Crain beat almost impossible odds to save both planets? Crain has a plan that might work (at least in the days of pre-ecological awareness, not so much in the 21st century).
They seldom write them like this nowadays.