In February 2004 a Swedish journalist handed a publisher a tatty plastic bag. In it was a manuscript for a thriller about a punky girl, computers and the corrupting effect of money on human behaviour. Eight months later, Stieg Larsson fell down dead, never to see The Girl with the Dragon Tattoo or its two sequels in print — or to know that he would become one of the world’s bestselling and most lucrative writers. Nor would he ever be aware that the woman he loved and lived with for 30 years would be, now five years to the week since his death, still locked in a lonely battle against his brother and father for a penny of his new millions.

The story behind the thriller of the year — the third book in the trilogy is set to become the Christmas bestseller — is as intriguing as one of Larsson’s stories and mirrors many of his gripping themes. Leif G.W. Persson, a Swedish crime novelist, summed it up when he wrote that the case is “a legal aftermath that is so unpleasant, not even Larsson could have thought it up”.

Larsson was reportedly estranged from his brother and father for much of his life, although , through his death, he made their fortune. A quirk of his career meant that he never married his life partner, Eva Gabrielsson — and so, according to Swedish law, she had no entitlement to or control over his money or artistic future.

Larsson did not marry Gabrielsson to shield her from his sometimes risky work as a campaigner against fascism; but in doing so he failed to protect her from one of the most poisonous estate wrangles in recent years.

Last week there was a new twist in the saga when Larsson’s brother Joakim and father Erland announced to the Swedish press that they were offering his partner a limited “no strings attached” lump sum.

“But she has to call and say ‘yes please’,” said Joakim Larsson to a Swedish newspaper. Gabrielsson would not comment on what was involved in the latest offer: to barter through the media, she said, would be “too low”. And what is the ace in her hand that Stieg Larsson’s family are so keen to get hold of? Appropriately, given Larsson’s tech-obsessed plots, it is the laptop that the thriller writer left with Gabrielsson. It contains several hundred pages of a fourth novel and is now held in a high-security safe.

Gabrielsson is now at work on her own book, a memoir tentatively entitled The Year After Stieg. She was closely involved with the research during the production of Larsson’s so-called Millennium trilogy, and it is not thought that the book will stint on the famous Larsson invective.

Jan Moberg, a Norwegian publishing executive who has taken up the campaign for Gabrielsson, said that Larsson would be furious. “It’s like the plot of a Larsson novel,” he remarked when he set up the public appeal for funds for her legal case. “He wrote about how women are abused by men and how they sometimes fight back.”

Gabrielsson said at the time of Moberg’s appeal that she was grateful for the support from readers of Larsson’s books. His fans have rallied to her cause in a way that they feel will honour the memory of Larsson, a journalist who spent his life campaigning against racist groups through his own tiny magazine, Expo. The row has also led to calls for a change in the inheritance law in Sweden, where one adult in three has read the Larsson trilogy.

“Sooner or later I will run out of funds or become unemployed, especially in these times of crisis,” Gabrielsson said at the time. “What do I do then? Give up?”

First, a look at what is at stake. Larsson’s book sales in the UK have just reached the million mark and his first book, The Girl with the Dragon Tattoo, has now outsold the whole Dan Brown oeuvre in paperback in Britain. In 2008 he was the second highest-selling author in the world.

This adds up to a pile of gold: his estate is thought to be worth more than £20 million and that figure is set to double or triple over time. The second novel, The Girl who Played with Fire, was a huge success in America, where his last novel, The Girl who Kicked the Hornet’s Nest, has yet to be published. Brad Pitt and George Clooney are among many mooted for the role of the lumbering reporter, Mikael Blomkvist, in an inevitable Hollywood adaptation.

Larsson was born into a low-key northern Swedish family and raised by his grandparents until the age of 9. His grandfather, imprisoned during the war for his antiNazi beliefs, was a formative influence. It was a great shock for the boy when his grandfather died and he moved back with his father, Erland. He and Joakim shared a bedroom; Joakim recalls him always spinning tales, some lifted from Enid Blyton.

Erland bought Stieg a typewriter when his son was 12 but the incessant drumming into the night so upset the neighbours that he had to be moved into the basement. At 18 he met Gabrielsson at a rally against the Vietnam War, and they moved into a flat in Stockholm together.

Gabrielsson became an architectural historian, often supporting Larsson financially as he became a campaigning journalist on the far Left. He felt threatened as a result of this work and was careful to try to protect his partner. They avoided appearing together in public and their decision not to marry was one of self-protection: under Swedish law, married couples have to publish their address.

In 2001 Larsson began work on a crime novel inspired by the British and American thrillers that he had read since childhood. His heroine, Lisbeth Salander, was modelled on Pippi Longstocking — that is, if you can imagine Pippi with intimate piercings and an attitude. Blomkvist, her sidekick, may be based loosely on the author — passionately political and a fierce feminist.

He tapped away on the trilogy each night after work and in the holidays. The first publisher he showed it to rejected it. Then a friend recommended him to another publisher. The junior assigned to take a first look at the books said “when I started, I couldn’t stop reading” and they signed up the trilogy immediately.

Larsson thought that the books would be a modest success in Sweden and planned to keep writing more in the series, his editor Eva Gedin, at the Swedish publisher Norstedts, has said.

“He said that this would be his retirement fund. He wanted us to sell books so that he could become a writer full-time.”

But a few months later, a broken lift forced him to climb the seven flights of stairs to his office. He had a fatal heart attack at the age of 50; years of 60 cigarettes a day, coffee and workaholism are thought to be to blame (some conspiracy theorists disgree but most go with the results of his post-mortem examination).

When he died, they found that Larsson had made a will when he was young, leaving everything to the Communist party in his home town. But it was never witnessed, so instead the estate fell, under Swedish law, partly to the State and partly to his family. This included the half of the Stockholm flat that Larsson owned jointly with Gabrielsson, where she still lives.

Gabrielsson asserts that Larsson believed that she would be entitled to the money because she had the status of his common-law wife. But that status had no legal claim (as in Britain) and she has no recourse to the courts. Instead she must negotiate through lawyers.

In 2005 she stated that Erland and Joakim Larsson offered her Stieg’s half of their flat in exchange for his laptop.

“My legal adviser called it extortion,” she said at the time, although she has said that she has no plans to have this fourth book finished on his behalf.

Now the Larsson family has offered her 20 million kronor (£1.75 million) to settle the dispute, telling the Swedish newspaper, Svenska Dagbladet: “We have to move on.”

Gabrielsson maintains that Larsson was not close to his family: “Stieg distanced himself from them,” she said this year.

But Larsson’s father and brother are keen to prove that they were not as estranged as she purports. Erlan has said that he encouraged his son to “write something commercial” and that the trilogy was the result. His son shared the first drafts with him, he says. He adds that they have repeatedly invited Gabrielsson to be part of the decisions about Larsson’s work. Joakim reportedly told Stieg to make a proper will after his publishing deal.

But Larsson was careless with his health and his finances — as reckless, in fact, as a character in a thriller. And this leaves his last work as The Girl Who Fought For Years To Settle A Legal Dispute Over Her Lover’s Legacy. And, like his fourth book, even that is left unfinished.

Times recommends

• How to solve the big style questions
• Aids in Africa: ‘It is the staff who really inspire you’
• What's the best low-alcohol drink for Christmas?