"The Girl Who Takes an Eye for an Eye", the fifth Millennium novel, is due to hit bookstores on September 7 in 26 countries, including the United States, France, Germany and Britain.
"I'm waiting for the storm," Lagercrantz told AFP with a nervous laugh during a recent interview at his fashionable Stockholm apartment.
Swedish publishing house Norstedts has gone to great lengths to keep details of the latest instalment shrouded in secrecy, given what's at stake: the previous book, "The Girl in the Spider's Web" from 2015, also written by Lagercrantz, sold six million copies in 47 countries.
The first three books, penned by the late Stieg Larsson, sold 80 million copies in 50 countries.
Emotional and high-strung, Lagercrantz, 54, is full of contradictions: he at once fascinates, annoys and elicits sympathy, he's fond of superlatives and gesticulates wildly when speaking.
With the book's release date looming, he admits to having mixed feelings.
He's relieved at having finished the manuscript, but also terrified by critics, some of whom won't forgive him for taking over the series from compatriot Larsson, who died suddenly of a heart attack at age 50 in 2004 before the series gained global fame.
"There are a lot of translators who have just received it via an encrypted link, it's all very secretive. Now we're beginning to get some feedback about the book and, fingers crossed and touch wood, it seems promising."
A heroine with 'problems'
Very little has been revealed about the plot of the fifth book. As with the preceding tome, details are trickling out, drop by drop.
"All I can say is that I started out by putting her in prison, in the worst kind of women's prison, where she immediately encounters quite a few problems," Lagercrantz says without divulging any more.
In addition to Salander, readers will also reacquaint themselves with investigative journalist Mikael Blomkvist.
Lagercrantz says bringing Salander to life, with her troubled past, is a challenge for him -- he would have written an entirely different leading character.
"I would have created a softer heroine, someone nicer, more delicate and sensitive than Stieg Larsson did," he admits.
But he acknowledges she makes for a good read.
"Lisbeth's personality, her iconic personality, needs problems. So of course I have to give her tonnes of problems. And in some ways she's also suited to being an underdog."
And that, he says, is what readers will see in the fifth instalment, the second of three he's signed on to write.
Millennium was the brainchild of Larsson, a left-wing activist from a working-class family in Sweden's far north -- a sharp contrast to Lagercrantz's upbringing in Stockholm's intelligentsia.
Lagercrantz, meanwhile, rose to fame in Sweden in 2011 after penning football star Zlatan Ibrahimovic's official biography.
Kill Lisbeth Salander ?
After Larsson's death and the ensuing wild success of his trilogy, Norstedts decided -- with the agreement of his only heirs, his father and brother -- to continue the series with a new author.
Lagercrantz was recruited and the fourth book was generally well-received. With the fifth one, he wants to win over those unconvinced about his worthiness.
One of them is Eva Gabrielsson, Larsson's partner of 32 years until his death. The couple were not married and Larsson left no will, so his estate went to his brother and father. Gabrielsson lost a bitter battle with them to manage his work.
She has from the beginning been critical of the decision to continue the trilogy, slamming it as a purely money-making project and blasting the choice of Lagercrantz as author.
"That's the only shadow over this project, which has otherwise been so enjoyable," Lagercrantz said.
"If you think of Stieg Larsson's books, I know now, in hindsight, that it was good for his body of work" to continue the series.
"A whole new generation has discovered his books... and his characters," he said.
And rest assured, Lisbeth Salander will live on.
"She'll continue to live on in one way or another. Lisbeth Salander is not going to be killed off right away, because she's a person who somehow reaches into our hearts and souls."
As for Lagercrantz, what will he do after he's written book six?
"I'll move on and find a new challenge."
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