The Swedish writer commissioned by the estate of Stieg Larsson to the best-selling Millennium Trilogy has upped the sexual tension between the punk wild child, Lisbeth Salander, and extend crusading journalist, Mikael Blomkvist, in the stand-alone sequel.

But true to superhero mythology, author David Lagercrantz hints any desire between the two characters, like Superman and his chaste girlfriend Lois Lane, will never be properly realised.

The Girl in the Spider's Web was released Thursday morning in 27 countries worldwide on the 10th anniversary of the publication of The Girl with the Dragon Tattoo, the first in the three-book Millennium series.

Unprecedented secrecy characterised the writing of the manuscript, which Lagercrantz did on a computer without internet access to ensure there was no leak.

Larsson's widow, Eva Gabrielsson, has criticised the new publishing venture as craven.

"There is no money in the world that would compensate me for writing a lousy book," Lagercrantz told Fairfax in his only Australian interview.

Lagercrantz says he was given a free hand by the Larsson estate, controlled by Larsson's father and brother, to recreate the murky universe of right-wing extremists and covert surveillance, and regularly consulted real-life hackers for advice.

Set in the present day, Spider's Web sees Salander and Blomkvist join forces after a long separation to protect a Swedish scientist, his eight-year-old autistic son, and the scientist's life's work in artificial intelligence.

Blomkvist's Millennium magazine is suffering a funding crisis, and the real enemies to fear are not outlaw hackers but state surveillance agencies.

Salander finds herself the target of a group of ruthless cyber gangsters, known as the Spiders, while trying to hack into the American National Security Agency.

"It is very interesting what kind of relations [Salander and Blomkvist] have," Lagercrantz says. "At the end of book one Lisbeth Salander fell in love and Mickael Blomvkist, he's a womaniser but he feels sometimes like he's a bit of a father to Lisbeth, but he's certainly attractive.

"That's one of the exciting parts of the series: what's really going on between them? They are connected to each other, they've saved each other's life so that's the tension ... and I love that, and I think it should go on like that. Superman should never marry Lois Lane, they should continue to [go] back and forwards and we would never know."

Larsson died in November 2004 before The Girl with the Dragon Tattoo and its two sequels were published. Larsson's books went on to sell more than 80 million copies in 50 countries, putting them among the most successful crime novels in modern times.

Lagercrantz is the son of a late Swedish literary scholar, Olof Lagercrantz, and a descendent of Swedish nobility. He is best known as the ghostwriter of the critically acclaimed "autobiography" I am Zlatan Ibrahimovic.

"I want to be read and I certainly want to sell but I also see my father's eye from heaven: 'Always write quality. It doesn't matter if you sell, if it's good, it's good; if you capture the complexity of life'."

Read an interview with Lagercrantz's in Saturday's Spectrum