"The Ship from Atlantis" is a 1967 novel written by H. Warner Munn, and is a sequel to a book published almost thirty year's previously, 1939's King of the World's Edge. It was published as an Ace Double, the other side being The Stolen Sun.
The story is a mixture of sword and sorcery and science-fiction taking place in an alternate history. Specifically, it is about an Arthurian knight, Gwalchmai, who came to the New World to fight for the Aztecs against the Mound Builder civilization. And that is just the backstory, covered in the previous work. This book chronicles the return of Gwalchmai to Europe. Or his attempt therefore, because while sailing back, he gets trapped in the Sargasso Sea, and, as the title suggests, finds a ship from Atlantis. He also discovers that Atlantis, centuries ago, had an odd event where certain members of its population were turned into violent psychopaths, who were then sequestered on an inescapable island. He learns this from Corenice, a once human woman whose body is now in a magical bronze statue. Gwalchmai and Corenice, together with the ship, which is also a magical/high tech dragon robot, end up fighting the violent, mutant Altanteans. The result is never really in doubt, since, just for starters, Corenice is an indestructible magic robot. It was a fun fantasy novel where the story was resolved by adventures, not by dramatic decisions. It didn't really introduce new concepts or ideas to the science-fiction world.
But that is my review, reading the book in 2021, 54 years after its publication, and 96 years after Munn published his first science-fiction story. Of late, we are spoiled: science-fiction and fantasy are mainstream things, and movies with fantasy or science-fiction plots can make over a billion dollars. In fact, they are the only movies that can. This book presents the idea of a Celtic warrior visiting North America and then teaming up with a magical robot to defeat a colony of insane Atlanteans. At the time it was published, this cavalcade of ideas would have probably been novel and entertaining, even without presenting any thought-provoking concepts.