The American novelist, best known for writing To Kill a Mockingbird, has passed away aged 89.

The American author Harper Lee, best known for writing the novel To Kill a Mockingbird, has died aged 89. A statement released by Lee’s attorney, Tonja Carter, said: “Lee has passed away early this morning in her sleep,” adding that her death was unexpected.

Lee’s seminal novel, about Atticus Finch, a respectable lawyer who confronts the ingrained racial prejudices in the Southern United States when he defends a black man accused of raping a white woman, is considered to be one of the most significant contributions to American literature of the past century. The author received one of America’s highest honours, the Pulitzer Prize, for To Kill a Mockingbird, in recognition of her valuable study of race in American society.

For decades it seemed that this extraordinary book would be her sole literary accomplishment until Lee shocked the world by releasing a sequel, Go Set a Watchman, in 2015. The surprising appearance of Lee’s second novel became the literary sensation of the year, and smashed sales records in the U.S., with over 1.1 million copies of the book selling in its first week.

Born in April of 1926, Lee was the youngest of four siblings. Like her young heroine in To Kill a Mockingbird, Scout, she was a proud tomboy from birth, and perhaps because of her reluctance to fit with the domestic female stereotype, Lee pursued an academic, intellectually rigorous path in life. She studied to become a lawyer in Alabama, but shortly before graduating in 1952 she moved to New York to become a writer.

The success of her debut novel, published in 1960, was astronomical, but Lee was never one to meet expectations, and despite her notoriety, she never actively sought the limelight. She was a publicity-shy and enigmatic character. She never married, rarely gave interviews, and preferred to live in private seclusion, either in New York or Monroeville, Alabama in America’s Deep South, where she grew up and also spent her final years after suffering a stroke in 2007. However, her mental acuity was never blunted, according to her agent, Andrew Nurnberg. “When I saw her just six weeks ago, she was full of life,” he said in a brief statement. “She was quoting Thomas More and setting me straight on Tudor History.”

Lee’s life was also shrouded in controversy. She shared a close friendship with the iconic American writer and essayist, Trueman Capote, who she had known since childhood. Conspiracy theories circulated that Capote was the actual author of the To Kill a Mockingbird, and while never substantiated, an unresolved falling out between the two friends added fuel to the fire of this rumour. Much later in her life, accusations that her lawyer, Tonja Carter, had manipulated Lee into publishing her second novel also percolated through the literary community, although an investigation by Alabama state officials found no evidence of coercion.

To Kill a Mockingbird has gone on to be required reading in schools across the world. It is estimated to have sold over 30 million copies worldwide, been translated into 40 languages and is one of the most enduring best-sellers ever penned.