Nelle Harper Lee, the Pulitzer Prize-winning author of the best-selling classic "To Kill a Mockingbird" has passed away, her publisher and city officials in Monroeville, Alabama, confirmed. The celebrated writer was 89 years old.

Lee died peacefully on Thursday, her publisher HarperCollins said in a statement, without giving any further details on her death.

"The world knows Harper Lee was a brilliant writer, but what many don't know is that she was an extraordinary woman of great joyfulness, humility and kindness. She lived her life the way she wanted to - in private - surrounded by books and the people who loved her," Michael Morrison, head of HarperCollins U.S., said in the statement.

'Shocked' by success

"To Kill a Mockingbird" quickly became a best-seller following its 1960 debut. It tells the story of a girl called Scout growing up in a Depression-Era town in Alabama. Despite numerous threats, Scout's lawyer father defends a black man wrongly accused of raping a white woman.

The book's child's-view of racial injustice won a Pulitzer Prize in 1961 and was made into an Oscar-winning movie in 1962.

The author herself couldn't believe the book's success.

"It was like being hit over the head and knocked cold," she remarked during a 1964 interview - at a time when she still talked to the media. "I didn't expect the book to sell in the first place. I was hoping for a quick and merciful death at the hands of reviewers, but at the same time I sort of hoped that maybe someone would like it enough to give me encouragement."

In 2015, the book's worldwide sales were reported by HarperCollins to be more than 40 million.

When the Library of Congress did a survey in 1991 on books that have affected people's lives, "To Kill a Mockingbird" came in second only to the Bible.

Second and last book in 2015

Following the book's success, Lee began retreating from the public eye and moved back to her Alabama hometown. She spoke sparingly to the press, deflecting interviews for decades.

She published no other book until shocking the world in 2015 by permitting her book "Go Set a Watchman" to be released.

Mourning 'Mockingbird' on social media

Many book-lovers took to Twitter following the news of Lee's death to reflect on the author's life.

Like Apple CEO Tim Cook:

And American TV anchor Katie Couric:

Some made jokes about Lee's reclusive attitude towards the press:

Others shared their favorite quotes, like "Selma" director Ava DuVerney and the Library of America:

rs/jm (AP, AFP, dpa)