"I am lucky to have written as many books as I have, really, and it has been a joy. With old age, it becomes very difficult. It takes longer for the inspiration to come, but the thing about being a writer is that you need to write."
It appears likely that the book was not completed. Lady James's literary agent, Carol Heaton, said: "As far as I know, it was very much in her head but I don't think she got very far. I certainly haven't seen it."
She said it was "extremely unlikely" that a publishable manuscript would come to light.
Lady James had a fear of leaving an unfinished manuscript. To do so would be "intolerable", she once said.
She was also firmly of the belief that if her story was not up to standard, she would not submit it for publication.
Her most famous character was Adam Dalgliesh, a poetry-writing policeman from Scotland Yard, introduced in the 1962 novel Cover Her Face.
His 14th and final appearance was in The Private Patient in 2008.
Lady James published her last novel, Death Comes to Pemberley, in 2011. It was a murder mystery sequel to Pride and Prejudice, combining her two great literary passions: crime fiction and Jane Austen. The BBC adapted the story last Christmas.
News of her death prompted a flurry of tributes from fellow authors, including the crime writers Ian Rankin, Val McDermid and Ruth Rendell.
Baroness Rendell, a close friend, said: "She was wonderfully accurate in her police work. She really took great pains about it. She did not make mistakes; she saw to it that she didn't."
Rankin praised Lady James's "sharp intellect" and "ready wit", while McDermid - referring to Lady James by her Christian name - said: "Today, I've lost a friend as well as a teacher. There was nothing cosy about Phyllis."
Mark Thompson, then director-general of the BBC, discovered that in 2009 when he agreed to be interviewed by Lady James on Radio 4.
Drawing on her time as a BBC governor, Lady James accused him of presiding over a corporation rife with ageism, dreadful programmes and executives paid "vast sums of money".
Another author and friend, AS Byatt, said Lady James was successful because her crime novels were rooted in reality.
"When people in her books died, the other characters' lives changed as they would in real life. Phyllis was working with real people that she cared about," Byatt said.
Lady James is survived by two daughters, five grandchildren and seven great-grandchildren. Her husband, Connor White, died in 1964.
Read more: 10 things you might not know about crime writer PD James