The 1949 exorcism that inspired William Blatty to write The Exorcist, recounted in admirably restrained and documented fashion by an unlikely source: military-expert Allen (Merchants of Treason, 1988, etc.). Unlike Blatty's possessed teenage girl, 14-year-old Robbie Mannheim (a pseudonym) of Mt. Ranier, Maryland, doesn't swivel his head like a top or levitate. But when fruit and then a vase fly through the air in his presence, his middle-class parents call on an M.D., a psychologist, and finally a minister for help. The minister suspects a poltergeist, but when bloody scratches appear on Robbie's body, the reverend tells the family, ``You have to see a Catholic priest. The Catholics know about things like this''- -advice that leads the Mannheims to a local priest whose exorcism of Robbie aborts when the boy slashes him with a mattress spring. The distraught parents take their son to St. Louis, where they meet Fr. William S. Bowdern, a 52-year-old Jesuit attached to St. Louis University. It's Bowdern who conducts the successful weeks-long exorcism, involving nightly incantations by the priest and several assistants as Robbie--who claims to be possessed--spits, urinates, writhes, cackles, and manifests words in blood (``HELL''; ``CHRIST'') on his body until the ``demon'' departs shortly after Easter. To his credit, Allen reports the more sensational aspects of Robbie's ordeal with a poker face, focusing instead on the spiritual and emotional issues involved, providing brief histories of the Jesuits, poltergeists, and possession. In an afterword, he weighs--without judging--the likelihood of Robbie having been possessed, and he discusses his sources, including one eyewitness and, crucially, a hitherto unrevealed daily journal of the exorcism kept by one of Bowdern's assistants. One can't blame Blatty for sleazing up Robbie's plight, but it's good to have Allen's levelheaded account, which allows the apparent facts of this influential case to speak for their own--and compelling--selves.