Unseen research by the late Stieg Larsson into the assassination of Swedish prime minister Olof Palme is set to be revealed in a new true crime book.

Larsson is most famous for his bestselling Millennium series of thrillers that explored the dark underbelly of Swedish society and politics. A journalist for his much of his life, he died suddenly in 2004, just months after selling his first book, The Girl With the Dragon Tattoo. He left behind completed manuscripts for the two sequels, which have together sold 80m copies around the world.

In 2014, it was revealed that, right up to his death, Larsson had been researching the unsolved assassination of Palme, leaving behind 20 boxes of research in his private archive. A populist, leftwing politician, Palme was shot in 1986 after leaving a Stockholm cinema with his wife. The assassin’s identity was never uncovered, but the case made international headlines again in 2014 when it was revealed that Larsson, by then a posthumous bestseller, had once identified a potential suspect to police, although the case was not pursued.

Larsson, who tracked far-right groups in Sweden for years, was targeted by them in turn. He never married his partner of 32 years, Eva Gabrielsson, as their address would then be logged publicly, as per Swedish law. However, this led to a bitter legal fight over Larsson’s estate, which was handed to his brother and father over Gabrielsson.

In 2014, Gabrielsson revealed that they had spent most of the year after Palme’s murder attempting to identify the killer together.

On Wednesday, Amazon’s literature in translation imprint Amazon Crossing, announced it had acquired The Man Who Played With Fire by Jan Stocklassa, translated by Tara F Chace. Stocklassa, who also produced a recent documentary of the same name about Larsson’s research into far-right groups, began to access the latter’s archives in 2014. The book, according Amazon Crossing, contains “new facts about the case and reveals the hitherto unknown research … in a fascinating true crime story”.

Senior editor Elizabeth DeNoma said it was “about one of the most gripping unsolved murder mysteries of modern times, investigated by one of the most well-known authors of all time … We can’t wait to share this story with Stieg Larsson’s English-language fans who will be struck by the parallels between the famous author and his famous character, Mikael Blomkvist.”

Around 130 people have confessed to the killing of Palme over the last 30 years, with troops of amateur sleuths gaining their own Swedish slang term: privatspanare, or “private scouts”. Various theories, involving Yugoslavian security services and apartheid-era South African agents, have never been proven.

The book is set for publication in October.