It’s the story of John Wainwright, a child prodigy of extreme intelligence who eventually develops telepathy. John eventually discovers some other mutant superhumans like himself, and founds a colony on an island. Here’s a summary of this last segment of the book from Wikipedia (includes book spoilers):
# The Skid’s First Voyage. They travel to the South Pacific, picking up many colleagues on the way. On arrival they take over an island, massacring the original inhabitants.
# The Colony is Founded. John and Lo briefly return to England.
# The Colony in Being. The narrator travels to the island and describes the colony.
# The Beginning of the End. The island is discovered by a British surveying vessel called the Viking. This is followed by a visit from two British light cruisers, a visit from the Soviets, and a final attempt at mass arrest by an international expedition. All are repelled with psionic attacks.
# The End. Mercenaries invade the island, but are driven away with harsh psionic attacks. On 15 December 1933, the colonists deliberately destroy their own island.
Just for fun, more book covers for Odd John: