Dark ghost stories are not my favourite genre and Beyond Black is definitely not the sort of book I would normally pick up from the bookshelf, but for once I let myself be persuaded by the quotes on the cover. 'One of the greatest ghost stories in the language', said Philip Pullman; 'Something extraordinary' wrote Fay Weldon. With recommendations like this I had to give this novel a go, and when I did I found something quite strange - a mixture of madness, horror and fantasy with characters that were both real and unforgettable.

Alison is an unlikely heroine; middle aged, morbidly obese, often sweaty and stinking of death, plagued by sinister ghosts that nobody else can see - she makes her lonely way through life by working as a touring Medium. On stage she acquires an elegant, sparkling presence; her weight disguised by flowing robes and her manicured hands sparkling with gems and mood stones. Off stage, she is in constant pain - trying to keep the voices out of her head and to keep the terror of the afterlife from intruding into her everyday existence.

There are two people in Alison's life. The first and perhaps the most important is Morris, her deeply unpleasant spirit guide. Obsessed with sex and pornography, he constantly plagues Alison; fiddling with his private parts, trying to creep under her bedclothes-; his presence is constant and his voice is always in her head. The second person in her life is Collette, her acerbic and efficient assistant. Collette never sees Morris, but often knows that he is there by the faint smell of sewage that follows him around; she is scared of him, but at the same time, a faint doubt lingers at the back of her mind - are the spirits real, is Alison playing mind games, or is the answer far more disturbing?

Beyond Black follows the story of Alison and Collette as they tour the country to various psychic events and try to achieve normality by buying a large house on a modern estate. Alison cannot escape from either the demons of her spirit world, or from the real life problems that she seems to attract like a magnet. To discover the meaning behind the fiends that pursue her, Alison has to go back into her dark and half remembered childhood; a place where neglect by her prostitute mother failed to protect her from the spirits that she saw from an early age and the confusion of not knowing who was real and who had already "passed". To discover the truth about her present life, Alison must dig deep and try to untangle the mystery of her childhood.

Hilary Mantel has a rare skill; she manages to make the seediest of characters empathetic and lovable. I found myself simultaneously frustrated by Alison's inability to fight against her destiny, sympathetic to her impossibly hard childhood, and repulsed by her physical state. Either way I was utterly enthralled by the story and the personalities, desperate for Alison to fight back. The book has a lot of humour - if you like your humour black. The manifestation of the recently "passed" Princess Diana in her wedding dress, stamping her foot and shouting, "Why don't you just bog off" may not be everybody's cup of tea, but it was a very comic moment in the novel and one that I enjoyed.

Beyond Black is not in any way a traditional ghost story, but is utterly convincing. I was caught up in a narrative that was just as much about the relationship between two very different women as it was about the dark mystery of the spirit world. Although all the critics seem to love this book without exception, reviews from the reading public are clearly divided; some love it but many hate it. Did I enjoy it? I find it difficult to decide. I felt hugely frustrated as I was reading it; the pace of the narrative was very slow at times and I can understand why so many readers seem to have given up before the end.

Hilary Mantel is an English writer who is better known for historical fiction and personal memoirs. Her work is critically acclaimed and has been shortlisted for many literary awards.

Beyond Black was shortlisted for the Orange Prize in 2006.

My paperback edition was published by Harper in 2005. At 451 pages it is not a short or an easy read.