Even putting aside our current fondness for astrology and all things New Age, it is hard to grasp just how popular spiritualism was in the first half of the 20th century. By the middle of the 1930s, the various spiritualist churches in Britain had about 250,000 members, all eager to contact their relatives on the other side. They were aided in this task by legions of "materialisers", physical mediums who conjured up the disembodied voices - and sometimes the ghostly figures - of the long dead and buried. Unlike mere clairvoyants, who tended to be genteel, owlish spinsters, the best materialisers were women of "gross material nature" - stout creatures who were physical in every sense of the word.
Helen Duncan, who in 1944 was the last woman to be tried and convicted under the Witchcraft Act 1735, fitted this mould perfectly. A working-class mother of six, she was coarse and corpulent (at one point, it is said, her weight swelled to 25 stone), a fishwife with a foul mouth and a fondness for cigarettes and whisky. Her upper-middle-class fans - and she had many - did not want Duncan sipping sherry in their drawing rooms but, once the lights were dimmed, her talents were such that they were prepared to forgive their star turn her ill-breeding. At one seance, she reputedly produced more than 12ft of luminous matter, or "ectoplasm", from her mouth. She had to be seen to be believed.
Duncan, whose childhood nickname was Hellish Nell, was born in Callander, Scotland. Her husband, Henry, had been invalided out of the army, and the family was poor, subsisting on porridge, vegetable broth and stovies. They were saved from destitution, however, by Helen's "gift". Having foretold the imminent demise of her mother-in-law, Duncan began passing messages from those lost in the Great War to their grieving loved ones. Emboldened by her successes (some mediums could earn the equivalent of £500,000 a year), her performances became ever more exaggerated. Ectoplasm spewed from her mouth, nostrils, nipples and from between her legs, and formed itself into a garish troupe of spectral visitors who were introduced by a waspish spirit guide called Albert. Rubber gloves, faces cut from magazines, safety pins, strings, rods and yards of muslin - all of these were used as backstage props.
As a result, Duncan began to attract the attention of a growing number of sceptics. Those out to get her - including the influential Society for Psychical Research - became preoccupied with how she managed to hide so much "ectoplasm" about her person. Like many mediums, Duncan had regular nosebleeds, so it seems likely that some of it, at least, was stuffed up her nostrils. Then there were her other orifices: the ectoplasm was reported to be exceedingly smelly, "like ripe gorgonzola". However, stern tests (before official seances, she was stripped and searched) served only to strengthen the faith of her true believers; to them, every accusation of fraudulence was confirmation that her powers were as much feared as they were admired.
Even so, the authorities were after her. The police raided a seance, and Duncan was remanded in custody. At her trial, dozens of the faithful lined up to provide the court with accounts of the miracles she had worked. But the celebrated medium was condemned to spend nine months inside Holloway Prison. Outside, meanwhile, the tabloids thrilled to her story, but only the News of the World was sympathetic to her cause: the legal prosecution of witches and the war to liberate Europe, it said, were incompatible. Her sentence over, Duncan seemed exhausted and diminished - although the seances soon started up again. However, after another police ambush during a seance in 1956, she fell ill. A few weeks later, at the age of 59, she died.
Malcolm Gaskill's book is long and detailed and full of quirky detours into spiritualism's uniquely odd hall of fame (all its friends and enemies are here, from Arthur Conan Doyle to Harry Houdini). But its chief fascination, I think, lies in the way it shows how the spiritualist movement, for a certain group of women, proved to be more liberating than winning the vote.
It was possible to become an effective and wealthy medium irrespective of one's class or education. In the seance room, those who would otherwise have been expected to remain quietly at home found themselves the centre of attention, the menfolk in their thrall. Once entranced, Duncan could swear with alacrity, and be congratulated for it. Profanity, blasphemy, exhibitionism - all these small acts of rebellion against the social order were permitted while she was in touch with the world beyond.
The effect on Duncan, and on women like her, must have been intoxicating - in more ways than one. Look at photographs of mediums from this era and you will see them in poses of ecstatic abandon. The liberation to be found at seances was not just monetary; it was sexual, too. Considered in this light, it seems appropriate that Duncan served her sentence in the prison where, some years earlier, the suffragettes had been force-fed during their hunger strike. Perhaps Callander Community Council, which has yet officially to acknowledge its most infamous daughter, should stop thinking of Helen Duncan as a corrupt old crone, and start regarding her as a feminist icon instead.
Rachel Cooke is deputy editor of Nova magazine