After the death of Stieg Larsson, the author of “The Girl With The Dragon Tattoo,” the writer’s estate faced a daunting task – find a new voice for one of the bestselling series in modern literature.

When they first approached Swedish writer and journalist David Lagercrantz, his response wasn’t exactly excitement.

“I was absolutely terrified,” Lagercrantz told CTV’s Canada AM Friday morning. “I had nightmares of the fans coming after me, the critics coming after me --- and even (series protagonist) Lisbeth Salander coming after me, telling me, ‘You’re not doing me justice, David. I’m tougher than this.’”

Those initial nerves weren’t out of place. According to recent estimates, “The Girl With the Dragon Tattoo” and its subsequent two books, “The Girl Who Played With Fire” and “The Girl Who Kicked The Hornets’ Nest,” have sold more than 80 million copies since the first novel hit shelves in 2005. They have also been translated into several languages and spawned films in English and Swedish.

Lagercrantz was already signed by a publishing house when a competing publisher approached him with the idea of writing the fourth book, and he says he couldn’t pass up the opportunity to discuss the project. They spoke in a clandestine meeting at the rival publisher’s basement, and despite his early fears, Lagercrantz says his love of Lisbeth Salander and the prospect of keeping the celebrated series alive were enough to win him over.

“I still remember walking home in a sort of fever,” he said. “I [understood] that if I say ‘no’ to this, I will regret it my whole life.”

Which is how Lagercrantz came to write “The Girl in the Spider’s Web,” the fourth novel of the Millennium series, which hit shelves Aug. 27. It quickly rose to the top of Maclean’s fiction list, where it currently sits in first place, and it ranks second on the New York Times hardcover fiction list, behind Lee Child’s “Make Me.”

It’s the first book in the series not penned by Larsson, and Lagercrantz says he worked with his publisher to develop the storyline.

“They didn’t just say, ‘Go on David, you find something,” he said.

He was inspired by a story he’d written as a journalist about an 8-year-old boy – a “savant,” he said – who couldn’t talk but possessed an incredible visual memory.

“I thought, this guy is sort of a mirror figure to Lisbeth Salander, and what would happen if a guy like this witnessed a murder or something horrible? And what would happen if the murderer finds out that this boy is a savant, and who will protect him if not Lisbeth Salander?” he said.

And just like that, he had a plot.

Lagercrantz and his publisher were careful to keep the premise under wraps. He worked on a computer that had never been connected to the Internet, and he used code words when discussing the project over email. Even translators weren’t allowed to use connected computers.

“I sort of lived in the same world that I was writing about,” he said.

Like many great books, controversy has followed the Millennium series. Larsson died of a heart attack before the first novel was published, and his partner of 32 years, Eva Gabrielsson, was left out of the estate. Instead, Larsson’s family was given the money and intellectual rights to determine the future of the series – including selecting its new author.

“The only thing that really troubles me [is] that this book makes her angry and sad,” Lagercrantz said. “So what I wanted to say to her, and I know now for certain, [is] that this is good for the legacy of Stieg Larsson, because now a new generation of readers are finding his book.”

The new book may also inspire more people to delve into Larsson’s non-fiction works, which tackled important issues such as racism and intolerance in modern-day Sweden, Lagercrantz said.

“He was such a hero, he was such a great writer, and now we are finding that out – again. And I think that’s lovely,” he said.