I once read a sci-fi novel (possibly it was a pair of novels) written in the form of letters, memoranda, and reports at an institute (or two competing institutes?) where two psychics were being studied. The climax came when the two battled it out as personifications of good vs evil, and because the battle was psychic everyone who saw it contextualized it in terms of their metaphysical beliefs - Jesus vs Satan, Kali vs Vishnu, etc.
The only other scene I remember clearly involved one of the subjects keeping hundreds of balls bouncing in a handball court and the scientist computing that it was physically impossible for him to be in the room without the balls passing through the space occupied by his body.
I've always thought that the novel(s) were definitely epistolary, but since I've been looking online for six years with no luck, maybe they weren't.
I read them in grade school so they would have to have been written no later than the mid-80s.