I read this somewhere in the early 1990s in a science fiction anthology, I think. The protagonist is a man with a young son. He learns that some of the books that his son has are written in language that he and his son can understand, but most people cannot. He eventually meets the people behind the books and it's explained that the books are written in a way such that only mutants can comprehend it, but that they all also had further talents. He's tested, and it's revealed that his only talent is being able to read the script. Near the end of the story, all of the mutants are going somewhere else (another planet?) and he's told that he cannot join them, but that they would provide a replacement for his son. It ends with him and his wife meeting the simulacrum and finding it indistinguishable from the real thing.
The only other details I recall about it were that the place where the books came from could only be seen by mutants as per the language bit.