The young man had slowly become aware of his enigmatic memories, of otherworldly beings lurking in his life, of "strange coincidences" and time out of joint. What was happening? Who could tell him? Casting about for help, says the boyish Pennsylvania health-care worker, "I saw this article in the newspaper about Dr. Mack. And I thought if you can't trust a Harvard professor, who can you trust?"

John Mack is more than a Harvard professor; he is a respected author (his book on T.E. Lawrence, A Prince of Our Disorder, won the Pulitzer Prize in 1977), a psychiatrist who helped found...