Forty years ago this month, Flowers in the Attic, the debut novel of one VC Andrews was published. A review in the Washington Post didn’t mince its words, describing it as “deranged swill” that “may well be the worst book I have ever read”.

The plot is nothing if not deranged – but even 40 years later, utterly compelling. Cathy and Chris Dollanganger and their little brother and sister, Cory and Carrie, are beautiful, blond-haired and as happy as can be until their handsome father dies. Their destitute mother tells them that in order to regain the fortune from which she has been disinherited, they must hide in their grandparents’ vast attic for a few days so she can persuade their grandfather to reinstate her as his heir.

They end up trapped in the attic for more than three years, where poisoned doughnuts kill off one of them, and where the overheated drama somehow leads Chris to rape his sister – but it’s fine, says Cathy, “you didn’t rape me. I could have stopped you if I’d really wanted to.” Somehow, the incestuous teenagers acquire a romantic tinge: can Chris and Cathy escape, and will they make it as a couple despite everything that’s against them?

That “deranged swill” proved irresistible. Andrews’ gothic tale of incest, poisoning and dark family secrets has sold more than 40m copies. There have been two film adaptations and a stage play, as well as sequels, prequels and retellings penned by both Andrews and Andrew Neiderman, the ghostwriter who took on Andews’ mantle after her death. Flowers in the Attic inspired the band name for Amanda Palmer’s band the Dresden Dolls, while Gillian Flynn has said that the mother and grandmother characters begat her addiction to “wicked women”: “It felt so new and stunning to me – these witches who seemed quite real.”

‘Wicked sense of humour’ … Virginia Andrews

In her 1978 pitch letter, Andrews described her novel as a “fictionalised version of a true story” about “real children who struggled to survive under almost unendurable circumstances … basically a horror story”. Its editor, Ann Patty, has written of how on first encountering it, she stayed up “until 2am, reading it in one long gulp”. She told her friend, a literary agent who had described it to her as “awful and fabulous”, that it was “some sort of brilliant”.

Once the book was published in November 1979 with that iconic cutout jacket, readers quickly agreed. “I remember when Flowers first came out – teenagers everywhere were reading it and the more press it received about being ‘scandalous’ or ‘shocking’ only piqued interest, until it became a worldwide phenomenon,” says Jen Long, who now edits the VC Andrews franchise. “I think readers root for the children in general. We want them to escape and find their place in the world, where they never have to eat a powdered doughnut again.”

Patty and publisher Pocket Books realised they needed a sequel, and fast. Patty recounts in the Toast how she flew to Virginia to meet Andrews for the first time and discovered that she was in a wheelchair, “her blonde hair was carefully coiffed around her pale, flawless skin, her atrophied legs capped by pink slippers. Her blue eyes were wary.”

Born Cleo Virginia Andrews, the author was known as VC Andrews in the US to appeal to a male audience. (In the UK, she has always been Virginia.) The myths around her were myriad. A New York Times obituary said that she “kept her age secret, [and] was believed to be in her late 40s or early 50s when she died”. A People magazine profile in 1980 said that she “spent most of her life as an invalid in her Portsmouth, Virginia home, the victim of progressive arthritis”, standing at her desk to write. Andrews herself, in a 1985 interview with the writer Douglas E Winter, said that her handicap began when she was “coming downstairs at school when my heel caught on something, and I fell forward and twisted to catch the banister. Later, the doctors found that the twist had been very violent, and that it tore the membrane on my hip and started little bone spurs.

“A newspaper once said that I was ‘paralysed’. It made me really angry, because I am not paralysed. They think that if you are in a wheelchair, you are paralysed, or else you would be up on your feet. And I do walk; but since they don’t see me walk, they don’t think I can,” she told Winter.

The most terrible things about my childhood probably were those that I created in my mind Virginia Andrews

As an adult Andrews lived with her mother, to whom she dedicated her novel featuring one of the most dastardly fictional mothers ever written. As per her pitch, Andrews always claimed that her story drew from the life story of one of her doctors, not her own.

“A lot of people think I was tortured, but my parents didn’t do anything,” she told Winter. “The most terrible things about my childhood probably were those that I created [in] my mind because my childhood was so ordinary, and I wanted it to be more exciting.”

When Patty first met Andrews in her wheelchair, the author served her a plate of powdered sugar doughnuts as a nod to her murder plot. The pair discussed a potential sequel about Cathy and Chris. “Virginia had a wicked sense of humour, and we took turns throwing out plot ideas, devising new miseries for the children to face in the outside world – illness, suicide, obsessive love, sexual transgression, and madness.”

Petals in the Wind came out in 1980, followed by If There Be Thorns (1981) and Seeds of Yesterday (1984). Andrews took a step away from the Dollanganger series to tell the – possibly even more deranged – story of My Sweet Audrina (1982), about a girl whose family is haunted by the rape and murder of her mysterious older sister. It was her only standalone novel and a bestseller.

‘It was almost like I was possessed’ … Andrew Neiderman. Photograph: Simon & Schuster

Andrews died in 1986 of breast cancer. That same year Neiderman, whose books under his own name include hit The Devil’s Advocate, was approached by Andrews’ agent Anita Diamant. He had never read Andrews, but his wife was a fan. He had also been a high-school teacher so “was familiar with teenage girls”. When Diamant asked him to finish the Flowers in the Attic prequel Garden of Shadows, Neiderman said yes.

He began by analysing Andrews’ style. “In effect I was doing a research paper,” he says now. “First I looked at the syntax, the sentence structure. There were what I call VC-isms – she had her own way of describing things. I began to understand what she was doing with characters, how she’d get at them, their relations to their families.”

It’s the premises in VC Andrews novels, he believes, that make them work. “Imagine going into a movie studio and you say to the producers, ‘I’ve got a movie here about a mother who locks her children in the attic for three years and three months.’ The premise is what catches people’s interest so fast. In Heaven, for example, it’s the story of a father who sells his children,” he says.

“She’s intrigued by why people who are supposed to love each other hurt each other so much. When you get into all the motivations, the emotions, the jealousies, the problems between mothers and daughters, fathers and sons – you’re getting biblical if you think about it. Whenever there’s family, there’s going to be tension. To be able to see it, visualise it, understand it, is part of what makes VC Andrews so powerful.”

Garden of Shadows was another hit and Neiderman went on writing Andrews novels. For a time, he says, he’d write his own books on a different computer. “Often I was writing two novels at once, a thriller under my name and a VC Andrews novel,” he says. “In the early days when I used two computers, I would turn my chair around – it was almost like I was possessed.”

In 1990, with the publication of Dawn, a letter to readers revealed that “we have been working closely with a carefully selected writer to organise and complete Virginia’s stories and to expand upon them by creating additional novels inspired by her wonderful storytelling genius”. It was only in 1994, when the Andrews estate sued the Internal Revenue Service over taxes and Neiderman was a major witness, that it became clear to her fans for the first time that he was now Andrews.

The case helpfully lays out the precise appeal of Andrews’ books. “Each was written in the first person and featured, as its principal character, a teenager, generally female, who was failed, and in some way hurt, by an adult family member with a warped personality. In each, the protagonist overcame the adversities visited upon her or him and, at the end of the novel, faced a hopeful future.”

More than 30 years on and around 80 books later, Neiderman is still writing as Andrews, including a stage play of Flowers in the Attic and a new prequel – the first of a new trilogy – which follows the children’s great-grandmother Corrine Dixon. His name still doesn’t appear on the novels, aside from a reference to the “carefully selected writer”. He is, however, named online, both on publisher Simon & Schuster’s site, and his own, where he says that he “has brought the [VC Andrews] franchise from 30 million to over 106 million worldwide in 95 countries”. The secret isn’t a secret any more – although it’s hardly publicised.

A question mark remains over how much of the content is based on what she left behind. A rare interview with Andrews in 1981 stated that “the author, who is given to hyperbole, reports that she has 63 brief synopses of future novels tucked away”. Neiderman himself says he had text to work with for Garden of Shadows, “but most of it was inspired by things I saw in her writing”.

For now, Neiderman is writing three VC Andrews books a year. It takes him four months to write one, and he says he’s still enjoying it. “I get a passion for a premise. When I wrote about Satan coming to Earth and being a lawyer [in The Devil’s Advocate], that grabbed me and I loved writing about it. Some VC Andrews books have really seized me, like The Mirror Sisters, two identical twins where obviously the sibling rivalries are going to be really strong, that worked up into a nice series of books,” he says. “Or Celeste, about a mother who raises her daughter as a boy when the daughter’s brother dies on the farm, you have the whole sexual identity thing. I love those premises and from there you go on.”

But he’s not sure he has another 80 books in him. “That’s a lot,” he says. “I’ve 126 published novels already … [But] I work every day and enjoy it, it’s still fun for me.”