For more than half a century, “To Kill a Mockingbird” has stood apart as a singular American literary masterpiece, a perennial best seller that has provoked countless classroom discussions about racial and social injustice. It brought instant and overwhelming fame to its enigmatic author, Harper Lee, who soon retreated from the spotlight to her native Monroeville, Ala. She never published another book, leaving her millions of fans yearning for more.

Now, at age 88, Ms. Lee has revealed that she wrote another novel after all — a sequel of sorts to “To Kill a Mockingbird,” featuring an aging Atticus Finch and his grown daughter, Scout.

On Tuesday, Ms. Lee’s publisher announced its plans to release that novel, recently rediscovered, which Ms. Lee completed in the mid-1950s, before she wrote “To Kill A Mockingbird.” The 304-page book, “Go Set a Watchman,” takes place 20 years later in the same fictional town, Maycomb, Ala., and unfolds as Jean Louise Finch, or Scout, the feisty child heroine of “To Kill a Mockingbird,” returns to visit her father. The novel, which is scheduled for release this July, tackles the racial tensions brewing in the South in the 1950s and delves into the complex relationship between father and daughter.

Although written first, “Go Set a Watchman” is a continuation of the same story, with overlapping themes and characters. But Ms. Lee abandoned the manuscript after her editor, who was captivated by the flashbacks to Scout’s childhood, told her to write a new book from the young heroine’s perspective and to set it during her childhood.