Lucy Maud Montgomery submitted the manuscript of The Blythes Are Quoted on the day of her death. (National Archives of Canada/Canadian Press)

Many Lucy Maud Montgomery fans may be shocked next week when they encounter the dark themes in the final Anne of Green Gables book, but a literary scholar says it's a side of the iconic Canadian author that was always there.

Set for release on Tuesday, The Blythes Are Quoted is Montgomery's last instalment in her series about headstrong orphan Anne Shirley. It was long believed that The Road to Yesterday, published in 1974, was the final volume.

However, after poring through the author's archives at Ontario's University of Guelph, literary scholar Benjamin Lefebvre discovered clues that the tale was incomplete — missing many stories and poems that explored darker topics like death, murder, revenge, misogyny and even Montgomery's opposition to war.

Noting a reference in the author's 1942 obituary to her having dropped off a manuscript on the day of her death — now believed to be a suicide — Lefebvre investigated and discovered the document.

"This was submitted. This is not an incomplete work she left when she died," he told CBC News.

"She did finish it. She wanted it published, and so what is coming out is her final book in the form she wanted it to be published in."

'She was beginning to not be able to summon that lightness that leavens Anne of Green Gables and some of the earlier novels.'—Mary Rubio

The image long held by many Montgomery fans is of an author writing happy historical and romantic stories.

Lefebvre believes that the final book's darker tone — such a contradiction of fan expectations — was part of the reason the story was never published until now.

Montgomery's moods

The public's image of Montgomery began to change in the mid-1980s with the publication of personal journals that showed a very different side of the beloved Canadian writer, according to leading Montgomery scholar Mary Rubio.

"She was a person who had wide mood swings, depressive periods," Rubio said, adding that the author suffered poor health in her final days.

"At the end of her life, she was beginning to not be able to summon that lightness that leavens Anne of Green Gables and some of the earlier novels."

Last year, amid celebrations of the 100th anniversary of the release of Anne of Green Gables, Montgomery's granddaughter, Kate Macdonald Butler, revealed the long-held family secret that she had committed suicide.

Dark themes seen earlier

Initially of the belief that Montgomery had been trying a different approach with her final work, Lefebvre changed his mind when he reread her oeuvre.

"All of these [darker] elements are already there in a lot of her work, but pushed to the sidelines. What happens in the final book [is that] the secondary stories take over, so it's Anne and Gilbert that get pushed to the sidelines," he said.

"I'm hoping once the shock wears off people will go back and reread Montgomery and see something new, and see how Montgomery was trying to reinvent Anne."