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This will probably be a long shot, but I read a novel about twenty years ago that I'm sure is from the late sixties.
I can only really remember the main plot point: There's a man in it who can change reality at will with only his mind. A medical doctor studies him and uses this to his own ends. Two more things: there was an alien invasion and people were preoccupied with a volcano at some point.
Don't remember any character names or locations. Sorry about that. If I knew those, I'd probably be able to find this myself. My son and his wife are really getting into sci-fi books and stories and I'd like to be able to recommend this one to them, since I remember enjoying it at the time.