Read in the 1990s. I don't think it was by any of the famous SF authors.
I think the cover had a black-and-white image on a black background. Some of the book was told in the form of documents and interview transcripts.
The setting is contemporary or near-future for the 1990s. A man, I think a farmer from Eastern Europe, had an accident. When he woke up he was speaking in tongues and suddenly super-intelligent. He started some kind of cult and was the main antagonist.
One of the protagonists was a therapist who had a crush on a therapist who was telepathic. Another protagonist was some kind of scientist, and I think there was a third who was a special agent of some kind. They find a college kid who is also super-intelligent and seems to be able to bend reality. There's a bit where he likes to go to a squash court and bat balls against the wall. He gets up to over a dozen balls in the air. When the scientist tries to run simulations, he finds that it's impossible to keep so many balls going if they follow the laws of physics, and concludes the guy must have telekinesis.
Eventually they find out that both the college kid and the farmer are possessed by super-beings from the future, who were sent back in time as some kind of exile. The college kid had a serious illness as a baby (possibly measles), and that's when he got possessed. The one possessing the college kid is good; the one possessing the farmer is evil. There's a big showdown where the college kid's super-being exorcises the farmer, who wakes up very confused and only speaks his native language.
In the epilogue, the therapist is waiting at an airport when he meets the college kid again. The college kid makes him telepathic, and he's able to sense the feelings of the people around him.
It is not The Infinite Man or any of the prior novellas. There are some similarities to Ben Bova's Orion series, but in this book the future being is not the protagonist.