The Exorcist by William Peter Blatty (1971, Bantam Books) Vintage Paperback

Georgetown, Washington D.C. Actress and divorced mother Chris MacNeil starts to experience 'difficulties' with her usually sweet-natured eleven-year-old daughter Regan. The child becomes afflicted by spasms, convulsions and unsettling amnesiac episodes; these abruptly worsen into violent fits of appalling foul-mouthed curses, accompanied by physical mutation. Medical science is baffled by Regan's plight and, in her increasing despair, Chris turns to troubled priest and psychiatrist Damien Karras, who immediately recognises something profoundly malevolent in Regan's distorted fetures and speech. On Karras's recommendation, the Church summons Father Merrin, a specialist in the exorcism of demons . . .