Author Nelle Harper Lee, who won the Pulitzer Prize for fiction in 1961 for her book, "To Kill a Mockingbird," passed away in her sleep Friday morning at the age of 89, her family has confirmed.

"This is a sad day for our family. America and the world knew Harper Lee as one of the last century's most beloved authors," Hank Conner, Lee's nephew and a spokesman for the family, said in a statement Friday morning.

"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."

Conner's statement indicated that "Ms. Lee passed away in her sleep early this morning. Her passing was unexpected. She remained in good basic health until her passing."

Services for Lee have not been announced, but Conner said the funeral will be private as per her request.

Lee was born April 28, 1926, in Monroeville, the youngest of four children of lawyer Amasa Coleman Lee and Frances Cunningham Finch Lee.

As a child, Lee attended elementary school and high school just a few blocks from her house on Alabama Avenue. In a March 1964 interview, she offered this capsule view of her childhood: "I was born in a little town called Monroeville, Alabama, on April 28, 1926. I went to school in the local grammar school, went to high school there, and then went to the University of Alabama. That's about it, as far as education goes."

She moved to New York in 1949, where she worked as an airlines reservations clerk while pursuing a writing career. Eight years later, Lee submitted her manuscript for "To Kill a Mockingbird" to J.B. Lippincott & Co., which asked her to rewrite it.

On July 11, 1960, "To Kill a Mockingbird" was published by Lippincott with critical and commercial success. The author won the Pulitzer Prize for fiction the following year.

Lee's novel tells the story of small-town lawyer Atticus Finch of Maycomb, Ala.--based on Monroeville -- and his children, Scout and Jem. Told from Scout's point of view, the book reflects the innocence of children growing up in the early 1930s. It also depicts the various social classes that existed then, and brings the undercurrents of racism to light.

More than a half-century after its publication, the novel continues to be studied by high school and college students. It has sold more than 30 million copies--still selling nearly a million copies per year by the 50th anniversary of its publication in 2010, according to Publishers Weekly--and has been translated into more than 40 languages.

The film adaptation of the novel, with Gregory Peck as Atticus Finch and Mary Badham as Scout, opened on Christmas Day of 1962 and was an instant hit. It was nominated for eight Academy Awards and won four, including Best Actor for Peck and Best Screenplay for Horton Foote, who wrote the screenplay for the movie based on the book. Lee became close friends with both of them.

The novel also inspired a generation of lawyers with its portrayal of the gentle, wise Atticus Finch, who defends a black man, Tom Robinson, falsely accused of raping Mayella Ewell, a white woman. Meanwhile, the Finches' strange neighbor, Boo Radley, who strikes fear in Scout's and Jem's hearts, turns out not to be the monster the children expect him to be.

Though Lee denied that the novel was autobiographical, many parallels exist between "To Kill a Mockingbird" and Lee's own childhood. Her father was also a lawyer who owned the town newspaper. Comparisons have been made between Lee and Scout, the 9-year-old tomboy protagonist, especially in her friendship with Dill, a character widely considered to have been based on Lee's own childhood friend, Truman Capote.

When he was a child, the author of "In Cold Blood" often stayed with his cousins, who lived next door to the Lees. Capote and Lee collaborated on the early stages of his novel and remained lifelong friends.

The interior of the Monroe County Courthouse was reconstructed on a movie set in Hollywood for the film's pivotal courtroom scenes, and local actors bring the book to life each spring at the courthouse itself, where they stage "To Kill a Mockingbird" to sellout crowds.

The press-averse author was thrust into the public eye in February 2015, when her publisher, HarperCollins, announced the upcoming release of "Go Set a Watchman," Lee's second and final full-length published work.

The announcement almost immediately sparked rumors that Lee - who had long said she would never publish another novel - had been taken advantage of by her attorney, Tonja Carter.

In February 2015, the Alabama Department of Human Resources met with Lee at Meadows of Monroeville, the southern Alabama assisted living facility where she resided, and concluded that she was mentally competent to handle her affairs, but rumors persisted that she was not lucid enough to sign off on business decisions.

A guardedly private individual, Lee was respected and protected by residents of the town that displays Mockingbird-themed murals and each year stages theatrical productions of "To Kill a Mockingbird."

Lee returned to Monroeville for good once her beloved sister Alice became ill and needed help. She'd eat breakfast each morning at the same fast-food place, and could later be seen picking up Alice from the law firm founded by their father, which currently employs Carter.

Since she stopped granting interviews in 1964, Harper Lee has been fiercely protected by Monroeville residents. In addition to maintaining an apartment in New York City, Lee lived in her hometown with her sister, Alice Finch Lee, who is 15 years older and practiced law until she was 100 years old.

In recent years, Harper Lee had experienced declining health after a stroke left her partially paralyzed and confined to a wheelchair. She also had lost 95 percent of her vision, according to a 2011 interview with Alice Lee in the Press-Register.

AL.com reporter Michelle Matthews contributed to this report.