Okay, back in the early 80's I read a (pair of?) hardcover sci-fi novel(s) 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 contextualized it in terms of their metaphysical beliefs - Jesus & Satan, Kali & Vishnu, etc.
The only other scene I remember clearly involved one of the subjects keeping hundreds of balls bouncing in a handball court and an observing 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.
Finally, I have a feeling the author's name struck me as Eastern European.
I read them in grade school (I'm sure it all went over my head, but it was my custom to haunt the stacks of the public library reading anything with the little Atom sticker on the spine) and have been trying to ferret them out every year or so for the last ten years with no luck at all.