Novelist Nelle Harper Lee has died, ET can confirm. She was 89 years old.

"The family of Nelle Harper Lee, of Monroeville, Alabama, announced today, with great sadness, that Ms. Lee passed away in her sleep early this morning," Lee's family said in a statement on Friday. "Her passing was unexpected. She remained in good basic health until her passing. The family is in mourning and there will be a private funeral service in the upcoming days, as she had requested."

"This is a sad day for our family, " Hank Conner, Lee's oldest nephew and family spokesperson, also said in a statement. "America and the world knew Harper Lee as one of the last century's most beloved authors. We knew her as Nelle Harper Lee, a loving member of our family, a devoted friend to the many good people who touched her life, and a generous soul in our community and our state. We will miss her dearly."

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Lee was best known for her 1960 book, To Kill a Mockingbird, now considered to be a classic of modern American literature. The hugely successful novel, which deals with issues of rape and racial inequality in the Deep South of the 1930s, was awarded the Pulitzer Prize in 1961.

Lee, who was born and raised in Monroeville, Alabama, attended Huntingdon College in Montgomery and the University of Alabama in Tuscaloosa -- where she studied law for several years -- but did not finish her degree. In 2001, she was inducted into the Alabama Academy of Honor, and in 2006, she was awarded an honorary doctorate from the University of Notre Dame.

She was awarded the Presidential Medal of Freedom in 2007 by President George W. Bush, who called To Kill a Mockingbird "a gift to the entire world."

Last July, Lee's Go Set a Watchman, an earlier draft of To Kill a Mockingbird, was released to much frenzy. The book set a record for the highest adult novel one-day sales at Barnes & Noble, and Amazon stated that it was their "most pre-ordered book" since Harry Potter and the Deathly Hallows in 2007.

And despite Lee's literary classic coming out 55 years ago, her work is still being mined today. Last week, it was announced that screenwriter Aaron Sorkin is adapting To Kill a Mockingbird for an upcoming Broadway production, which will show during the 2017-2018 Broadway season.

"Like millions of others, I was saddened to learn this morning of the passing of Harper Lee, one of America's most beloved authors," Sorkin said in a statement to ET on Friday. "I'm honored to have the opportunity to adapt her seminal novel for the stage."

The novel already became a film in 1962, starring Gregory Peck as lawyer Atticus Finch.

Lee has also been portrayed twice in film -- once by Catherine Keener in 2005's Capote, and another time by Sandra Bullock in 2006's Infamous.

Interestingly enough, plenty of celebrity children have been named after characters in To Kill a Mockingbird, including Bruce Willis and Demi Moore's daughter, Scout Willis; Casey Affleck and Summer Phoenix's son, Atticus Affleck; and Daniel Baldwin and Isabella Hofmann's son, Atticus Baldwin.

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Celebs reacted to Lee's death on social media on Friday.