"Go Set a Watchman," which Lee completed in the 1950s and then set aside in favor of "Mockingbird," will be published July 14. It follows Scout, the little girl of "Mockingbird," as an adult.
The manuscript was rediscovered last year, Lee, 88, said in a statement from her publisher, Harper.
"In the mid-1950s, I completed a novel called 'Go Set a Watchman,' " she said. "It features the character known as Scout as an adult woman, and I thought it a pretty decent effort. My editor, who was taken by the flashbacks to Scout's childhood, persuaded me to write a novel (what became 'To Kill a Mockingbird') from the point of view of the young Scout.
"I was a first-time writer, so I did as I was told. I hadn't realized it (the original book) had survived, so was surprised and delighted when my dear friend and lawyer Tonja Carter discovered it. After much thought and hesitation, I shared it with a handful of people I trust and was pleased to hear that they considered it worthy of publication. I am humbled and amazed that this will now be published after all these years."
"Watchman" is set in the 1950s and is about Scout -- Jean Louise Finch -- returning to her hometown of Maycomb, Alabama (a fictional version of Lee's hometown of Monroeville), to see her father, the upright lawyer Atticus Finch.
"She is forced to grapple with issues both personal and political as she tries to understand her father's attitude toward society, and her own feelings about the place where she was born and spent her childhood," the press release said.
"To Kill a Mockingbird," which Lee wrote after she moved to New York, made her name. The book, published in 1960, won the Pulitzer Prize and was made into a beloved 1962 film. Gregory Peck won the Oscar for best actor for his portrayal of Atticus Finch.
It's a mainstay of high school reading lists and, as of 2006, had sold more than 30 million copies.
Lee, who returned to Monroeville several years ago, remembers being caught off-guard by its overwhelming success.
"I can't say that (my reaction) was one of surprise. It was one of sheer numbness. It was like being hit over the head and knocked cold," she said in 1964.
Until now, it had been her only published novel.
Jonathan Burnham, Harper's senior vice president and publisher, called "Go Set a Watchman" "a remarkable literary event."
"The existence of 'Go Set a Watchman' was unknown until recently, and its discovery is an extraordinary gift to the many readers and fans of 'To Kill a Mockingbird,' " he said in the statement. "Reading in many ways like a sequel to Harper Lee's classic novel, it is a compelling and ultimately moving narrative about a father and a daughter's relationship, and the life of a small Alabama town living through the racial tensions of the 1950s."
Sandy Smith at Monroeville's chamber of commerce said news of the book was "great news."
"Our main street director told me and she had heard it from the book store. It's definitely great news," she told CNN. "Everyone has always speculated about a second book. Everyone who read ('Mockingbird') has said, 'Oh I wish she had written another book.' It's good for Monroeville. Her book brings people here every day."
However, Smith honored Lee's desire to maintain a low profile. The author has been a quiet presence for years.
"Oh, I don't talk about Ms. Lee," Smith said when asked if she ever sees Lee around town. "Our policy is not to comment on Ms. Lee personally."