SINGAPORE - Swedish author David Lagercrantz wrote one of this year's most anticipated novels in a state of constant terror and what he calls a "psychological fever".
The 52-year-old is behind the sequel to the hugely successful Millennium crime trilogy, The Girl In The Spider's Web. Still keyed up despite a jam-packed schedule of up to 10 media interviews daily, he says: "I was in a fever while writing. I was terrified of not living up to Larsson, but it was a good thing. It helped me to concentrate better."
He is speaking to Life in a phone interview from Stockholm, Sweden, where he lives. The series' creator, Stieg Larsson, a Swedish writer and activist who campaigned for feminism and against white supremacy, produced the first three Millennium books. He died aged 50 of a heart attack in 2004.
Recounting the 18 months spent working on the sequel, Lagercrantz says: "The best hours to write were between five and seven in the morning, before I sent my kids to school. I would go for a walk, then maybe take a siesta. But all the time, I never stopped thinking about the plot."
The father of three credits his spouse, Swedish journalist Anne Lagercrantz, as the "first reader" for all his drafts.
"When we were dead tired, we would take a glass of red wine and discuss her day and my story plot," he recalls.
The book, which hit shelves worldwide on Aug 27, follows its protagonists, muckraking journalist Mikael Blomkvist and punk hacker Lisbeth Salander, as they work together with a professor in artificial intelligence to take on both cyber criminals and the United States' National Security Agency.
Titled That Which Does Not Kill You in Swedish, the 500-page tome will be translated into 42 different languages, and 2.7million copies have been printed. It is eclipsed perhaps only by Go Set A Watchman, the recently published blockbuster sequel to Harper Lee's debut novel To Kill A Mockingbird, which has 3.3million copies in print.
As of March, Larsson's three books - The Girl With the Dragon Tattoo, The Girl Who Played with Fire and The Girl Who Kicked The Hornets' Nest - have sold over 80 million copies worldwide combined. Lagercrantz was tasked to write the fourth part by the book's Swedish publisher Norstedts, as well as Larsson's father, Erland and brother, Joakim.
The franchise's success has yielded Hollywood and Swedish movie adaptations, with American actress Rooney Mara earning a Best Actress nod at the 2011 Academy Awards for her portrayal of Salander.
Lagercrantz also found his life imitating the shadowy, high stakes world of the thriller, as the project was cloaked in the sort of secrecy that even Edward Snowden would have approved of.
He wrote on a computer without an Internet connection, and hand-delivered the manuscript to his editor and managing publisher, who edited it on paper. Emails were kept to a minimum, and code words were used in such exchanges.
He says: "I lived in that same world, as I was writing the book.
"When Larsson wrote about Lisbeth Salander, she was an outlaw. But now, the worst hacking attacks are by the government, the intelligence officers, the NSA. We live in a world where we need her even more."
Despite having written crime stories and mysteries, he admits that crafting the story arcs was far tougher than what he has attempted before.
"With a conventional crime story, you've a corpse floating up, a detective with a drinking problem. He solves it, and it's over. But with Larsson, the intrigue is so complex. I had to weave so many threads together," he says.
His primary concern was adding depth to the character of Salander, the tortured genius of a heroine, adored by fans for her fearsome mien, embellished by an ensemble of body piercings, tattoos and goth-rocker outfits .
"I feel like readers should be able to feel her anger and pain between the lines," he says.
He adds that he was inspired by the Batman Begins film to "dig back" into her past, noting parallels between the two characters.
"I feel like she has a mythology, similar to Batman, whose parents were murdered. So I wanted to explore that deep wound to find out why she's such a great hacker."
Lagercrantz describes himself as an author who is "best writing in other worlds, colliding myself with the universe".
"I've always been interested in odd geniuses, like Lisbeth and Alan Turing," he says, referring to the prodigious British mathematician, whose demise formed the narrative thrust of his 2009 crime novel, Fall Of Man In Wilmslow.
A former crime journalist at a Swedish daily, Lagercrantz made his writing debut in 1997 with Ultimate High, the story of a Swedish adventurer Goran Kropp who climbed Mount Everest without oxygen tanks or sherpas.
He ghost wrote Swedish soccer star Zlatan Ibrahimovic's 2013 memoir I Am Zlatan, and made headlines in May for admitting to inventing quotes from interviews with the star, instead of quoting verbatim.
In his own defence, he said it had not been his intention to quote Ibrahimovic.
"If you want to find something that sounds true and authentic, the last thing you want to do is quote... I tried to get an illusion of him, to try and find the story. I tried to find the literary Ibrahimovic."
The Girl In The Spider's Web has also been tangled up in controversy - its publication was vehemently opposed by those close to Larsson, especially his partner of 32 years, Swedish architect and historian Eva Gabrielsson, 61.
As the couple never married, Larsson's estate, and the rights to any sequel, were handed over to his family.
Ms Gabrielsson has openly panned the choice of Lagercrantz as "idiotic", and said in an interview: "It's about a publishing house that needs money, (and) a writer who doesn't have anything to write so he copies someone else."
In response to this, Lagercrantz says: "The only sad thing for me about this was making Eva sad and angry. But I don't think good authorship should rest in peace. It is good for Stieg Larsson, for more people to read and discuss his life's works. They will become classics."
While he does not discount the possibility of writing more sequels (Larsson reportedly planned for six more), he says firmly: "I won't be Stieg Larsson for the rest of my life. I see my authorship as going into the worlds of others."
"This was one of the greatest challenges in my life, as there was so much pressure. But I am a reporter and writer. Once I have a good story that everyone says is important, it gets me going."
The Girl In The Spider's Web retails at $31.95 from major bookstores here.