MY HOME, NEW YORK CITY, is about the least spiritual place in the world and the last place you would expect to find yourself trembling with fear over anything as ethereal as demonic possession. New York should be the antidote to supernatural terrors. First, the place is never dark. Even at 2am the local streetlamps flood your bedroom - a bummer for bogeymen. Secondly, just as surely as every horror movie housing development is built on an Indian burial ground, Manhattan is built on commerce, from Dutch speculators trading pelts to Goldman i-bankers buying apartments for cash at bonus time.

This is not the stuff of campfire tales. So why did I find myself on a bright summer afternoon in the Village trying to calm my shaking hands as they slammed shut an old cloth-bound book?

The answer lay, in part, in the book. Its title was lurid enough - simply Satan - but its content should have been dull and dry: a collection of essays by Jesuits about the nature of the Devil. Yet the first pages sent a chill through me: “Satan, we must remember, has all the powers of an angel. Matter is nothing to him, he is pure spirit ...” The author went on to elaborate on the many potencies of a fallen angel to wreck a mere human being, just as the biblical Satan tortured Job. In a few phrases, the French Jesuit, as if with a wave of his hand, had laid low all the vanities of New York City. I felt cold, jittery with adrenalin. I nearly ran from the library.

Why do demons have the power to frighten us so? Most members of Generation X have a traumatic story about watching The Exorcist. I quailed before the trailer as a tot (Max von Sydow, in misty silhouette) and remember my sister, sleepless and weeping with fear, after seeing it at a slumber party. My memories were refreshed by the film's 20th anniversary re-release. I knew someone who could connect me to William Peter Blatty, the author of the original novel, by e-mail. Curious about revisiting childhood scares, I asked him about his sources for the book. His response: two lines, all capitals, two book titles. One was Satan; the other was an analytical study of possession by a German, T.K. Osterreich.

Through those volumes I traced the origin of many scenes in the famous movie. Like Bram Stoker with Dracula, Blatty had turned the demon into a literary monster. A demon has its powers - all the potency of an angel, the ability to move objects, read our minds; its victims - those curious about the occult, who might be open to invasion; its heroic adversaries - the plain-spoken exorcists chosen by the Church; and its weaknesses - an ancient Roman Catholic ritual. Blatty had extracted the lore and shaped his golem.

And yet for me this was more than entertainment. I felt drawn to the subject matter - partly through my upbringing in a nostalgic, smells-and-bells Episcopalian tradition that pined, perhaps perversely, for the unapologetic physicality of Catholicism. The blood, the flesh, the nails, the Cross. The reality of it. And why not the reality of evil, of demons?

Furthermore, my hometown in the Blue Ridge mountains of Virginia gave me an appreciation for the supernatural. It started with Aunt Betty. No relation - this was the honorific “uncle” and “aunt” of the South - she was a family friend who, in my childhood, owned a haunted house. There was a poltergeist that tore down curtains and flung flowers from their vases in full view of company; a ghost who appeared to the children at night as they lay in their beds.

Aunt Betty was able to determine (through consulting other townsfolk) that her ghost was the house's former owner. The offence, it seemed, was pure Virginia (a realm of garden clubs and historic landmarks) - Aunt Betty had ripped up the woman's boxwood hedge to put in a pool.

Aunt Betty's experiences gave credence to other supernatural stories, told by my father and originating in his hometown of Macon, Georgia. Houses where plates pulverised themselves in sinks; where shower doors slammed by themselves. Did these things happen only in the South? The linkages appeared elsewhere. The Gospels suddenly abounded with tales of exorcism (every town seemed to have a demoniac awaiting Jesus's cure). And in my own child's baptism - in the Greek Orthodox Church - I recognised in the liturgy (I could not quite believe it) an exorcism. The godparents ritually spit on Satan - “pah pah pah” - in order to cast him out and make way for the Holy Spirit.

And so this lore became material for a novel. For several years I immersed myself in this shadow world. While the writing was satisfying, I found myself too many times awake at night, pacing the apartment, checking the children ... and yes, crossing myself, and praying for protection. When after the final manuscript had been delivered my agent suggested I write “another book on demon possession”, I felt I had been kicked in the gut. No way, I told her. It would be like returning to the scene of a car accident.

In The Screwtape Letters, C.S. Lewis writes that man can make two equal errors, to disbelieve in devils, or “to feel an excessive and unhealthy interest in them.” I had strayed too close to the latter position. I had stood at the border of ordinary reality, leaned out, and listened to the snarls in the darkness. I gained experience enough to appreciate the danger - and the wisdom to be afraid.

A Good and Happy Child by Justin Evans
Vintage, £7.99; 336pp
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