Harper Lee, who wrote one of America's most beloved literary classics, "To Kill a Mockingbird," and surprised readers with a second book about racial injustice in the US South after living a largely reclusive life for decades, died at the age of 89 on Friday.

"To Kill a Mockingbird" was published in 1960 as the civil rights movement was heating up and its unflinching examination of racial hatred in the South made it especially poignant. Its theme could be summed up with the advice that Atticus Finch, the noble lawyer, gave his young daughter, Scout: "You never really understand a person until you consider things from his point of view - until you climb into his skin and walk around in it."

A statement from Tonja Carter, Lee's attorney in her hometown of Monroeville, Alabama, said Lee had "passed away early this morning in her sleep" there and that her death was unexpected. She would have a private funeral but no date was announced.

It had appeared that Lee's sole literary output would be "To Kill a Mockingbird," especially since she acknowledged she could not top the Pulitzer Prize-winning book. That was what made the publication 55 years later in July 2015 of "Go Set a Watchman" such an unexpected and somewhat controversial literary event.

In the first book, Finch, the adored father of the young narrator Scout, stood up to a white lynch mob and unsuccessfully defended a black man wrongly accused of raping a white woman. But in "Watchman," an older Atticus had racial views that left the grown-up Scout greatly disillusioned.

Lee reportedly had written "Go Set a Watchman" first but, at the suggestion of a wise editor, set it aside to tell a tale of race in the South from the child's point of view in the 1930s.

For many years, Lee, a shy woman with an engaging Southern drawl who never married, lived quietly and privately, always turning down interview requests. She alternated between living in a New York apartment and Monroeville, where she shared a home with her older sister, lawyer Alice Lee.

After suffering a stroke and enduring failing vision and hearing, she spent her final years in an assisted living residence in Monroeville.

"When I saw her just six weeks ago, she was full of life, her mind and mischievous wit as sharp as ever," her agent, Andrew Nurnberg, said in a statement. "She was quoting Thomas More and setting me straight on Tudor history."

The movie version of "To Kill a Mockingbird also became an American classic. It won the Academy Award for best picture in 1963 while Gregory Peck, who played Atticus and would become Lee's good friend, was named best actor.

SAD DAY IN MONROEVILLE

Spencer Madrie, owner of the Ol’ Curiosities & Book Shoppe dedicated to the work of Lee and other Southern authors, said Monroeville was in a sombre mood.

"You wish somebody like that could go on forever and be this lifelong legend," he said. "You don’t ever consider somebody like that passing, even though her legacy will last for generations after."

Monroeville, which inspired the town of Maycomb in the book, eventually took on aspects of a "To Kill a Mockingbird" theme park with statues of the main characters, murals of important scenes, a museum display and tours of the courtroom.

Lee's state of mind would become an issue last year when plans were announced to publish "Go Set a Watchman." Some friends said that after the death of her sister Alice, who handled Harper's affairs, lawyer Carter had manipulated Lee to approve publication.

Carter had said she came across the "Watchman" manuscript while doing legal work for Lee in 2014 and an investigation by Alabama state officials found there was no coercion in getting Lee's permission to publish.

A family friend, the Reverend Thomas Lane Butts, told an Australian interviewer that Lee had said she did not publish again because she did not want to endure the pressure and publicity of another book and because she had said all that she wanted to say.

Despite her private nature, Lee regularly attended an annual luncheon at the University of Alabama to meet the winners of a high school essay contest on the subject of her book.

In November 2007, she went to the White House to accept a Presidential Medal of Freedom from President George W. Bush, who at the time called her book "a gift to the entire world."

Bush said in a statement on Friday that he and his wife, Laura Bush, a former librarian, mourned Lee. "Harper Lee was ahead of her time and her masterpiece 'To Kill a Mockingbird' prodded America to catch up with her," he said.

White House spokesman Josh Earnest said Lee "had a way of telling stories that does have an influence and resonates with so many Americans." He said President Barack Obama had great respect for her.

News of Lee's death spread widely on social media and tributes poured in from well-known figures, such as Apple Inc Chief Executive Officer Tim Cook, who quoted the author in a tweet by saying, "Rest in peace, Harper Lee. 'The one thing that doesn't abide by majority rule is a person's conscience.'"

CHANGING RACIAL VIEWS

Nelle Harper Lee was born April 28, 1926, in Monroeville, the youngest of four children of A.C. and Frances Finch Lee and a descendant of Civil War General Robert E. Lee. Like Scout, Lee grew up a tomboy.

Lee had studied law at the University of Alabama but, six months before finishing her studies, she went to New York in the early 1950s to pursue a literary career while working as an airline reservation clerk.

In 1956 friends Michael and Joy Brown gave Lee a special Christmas gift, a year of financial support so she could work full time on "To Kill a Mockingbird."

An estimated 30 million copies of the book were sold. It would become required reading in many American schools but the American Library Association said it was frequently challenged by those who did not like its subject matter.

Lee also played a key role in researching another great American book by Truman Capote, her childhood friend and the inspiration for the frail, precocious Dill in "To Kill a Mockingbird."

In 1959 she accompanied Capote to Holcombe, Kansas, to work on "In Cold Blood," the chilling account of the murders of a farming family. Her mannerly, down-home approach undoubtedly smoothed the way for the flamboyant Capote.

There was speculation that Capote helped her write "To Kill a Mockingbird" but in his 2006 biography, "Mockingbird: A Portrait of Harper Lee," Charles J. Shields disputed that. He also said Lee's contribution to Capote's "In Cold Blood" was greater than believed.

Lee's sister said the authors eventually fell out because Capote was jealous of Lee's Pulitzer, which she won in 1961.

In 2006 Lee wrote a piece for O magazine about developing a childhood love of books, even though they were scarce in Monroeville.

"Now, 75 years later in an abundant society where people have laptops, cell phones, iPods, and minds like empty rooms, I still plod along with books," she wrote.

Celebrities, politicians saddened by author Harper Lee's death

US political and cultural figures mourned the death of author Harper Lee on Friday, crediting her with helping to promote tolerance and quoting her with admiration in social media and formal statements.

Lee, whose 1960 book "To Kill a Mockingbird" about racism and injustice in the US South is a classic of American literature, died on Friday at age 89 in her home town of Monroeville, Alabama.

"Harper Lee was ahead of her time, and her masterpiece To 'Kill A Mockingbird' prodded America to catch up with her," former President George W. Bush, whose wife Laura Bush is a librarian, said in a statement.

Bush, who awarded Lee a Presidential Medal of Freedom in 2007, said she had been a voice for tolerance.

Author Stephen King (@StephenKing) honoured not only Lee's work but the influence her talent had on another famed author.

"Let's celebrate the life of Harper Lee, who wrote an American classic and helped her friend Truman Capote write another," King tweeted Friday.

Alabama Governor Robert Bentley said that "it is because of Harper Lee that the world knows about her special hometown of Monroeville." The governor also noted in his statement the celebration in Alabama after the publication last year of Lee's second novel, "Go Set a Watchman" 55 years after the first.

Senator Rand Paul, a Republican from Kentucky, and Senator Richard Shelby, a Republican from Lee's home state of Alabama, both praised the Pulitzer Prize winning Lee as a great author.

"Today I join Alabamians and all Americans in mourning the passing of Harper Lee," Shelby said in a statement.

Admirers of Lee took to Twitter to post quotes from the author, with novelist Erica Jong and Apple Inc Chief Executive Officer Tim Cook joining the chorus.

"Rest in peace, Harper Lee. 'The one thing that doesn't abide by majority rule is a person's conscience,'" Cook wrote.

And Hollywood celebrities also expressed shock and sadness.

"Harper Lee my 1st favourite author!" tweeted Oprah Winfrey (@Oprah). "I always wanted to interview her. She said 'honey I already said everything I had to say.'"

"Oh no. The great Harper Lee has passed away," actress Debra Messing said on Twitter. "She changed the world with 'To Kill A Mockingbird.'"

Actor Josh Gad tweeted that "To Kill a Mockingbird" was the first book he remembers reading cover-to-cover. "It propelled me towards my love for lit," he said.

Seven facts about 'To Kill a Mockingbird' author Harper Lee

Harper Lee's "To Kill a Mockingbird" became one of the most beloved books in US literary history. Here are seven facts about Lee, who died at the age of 89:

* She went by Harper, her middle name, because she was afraid her first name, Nelle, would be mispronounced as "Nellie," not "Nell."

* Lee based the "To Kill a Mockingbird" character Dill on childhood friend Truman Capote, who in turn used her as the basis for a character in his "Other Voices, Other Rooms."

* "To Kill a Mockingbird," published in 1960, won the Pulitzer Prize the following year, but Conrad Richter's "The Waters of Kronos" beat her out for the National Book Award.

* Lee's fans were stunned to learn 55 years after publication of the novel of a long-stashed manuscript written before "To Kill a Mockingbird" and even more stunned that Atticus, the hero of the first book, was portrayed as a segregationist in "Go Set a Watchman." Atticus was based on Lee's father.

* Lee and actor Gregory Peck became friends during the filming of "To Kill a Mockingbird." She remained close to his family and Peck's grandson, Harper Peck Voll, is named for her.

* "To Kill a Mockingbird" created a cottage industry in her hometown, Monroeville, Alabama, with a museum dedicated to it, although Lee filed a lawsuit claiming it was selling unlicensed merchandise, such as "To Kill a Mockingbird" beverage coasters.

* Actresses Sandra Bullock ("Infamous" in 2006) and Catherine Keener ("Capote" in 2005) portrayed Lee in movies about the writing of Capote's "In Cold Blood."