The recently published 'Go Set a Watchman' is the long-awaited sequel to what book?

Go Set a Watchman is a novel by Harper Lee published on July 14, 2015, by HarperCollins in the United States and Willam Heinemann in the United Kingdom. Although written before her first and only other published novel, the Pulitzer Prize-winning To Kill a Mockingbird and initially promoted by its publisher as a sequel. It is now more widely accepted as being a first draft of the famous novel. The title comes from Isaiah 21:6: "For thus hath the Lord said unto me, Go, set a watchman, let him declare what he seeth." It alludes to Jean Louise Finch's view of her father, Atticus Finch, as the moral compass ("watchman") of Maycomb, and has a theme of disillusionment, as she discovers the extent of the bigotry in her home community.

The book's unexpected and controversial discovery, decades after it was written, together with the exceptional eminence of the author's only other book—an American classic—caused its publication to be highly anticipated; Amazon stated that it was their "most pre-ordered book" since Harry Potter and the Deathly Hallows in 2007, and stores arranged all-night openings from midnight to cope with expected demand.

According to the publisher, Go Set a Watchman, "...tackles the racial tensions brewing in the South in the 1950s and delves into the complex relationship between father and daughter." It includes versions of many of the characters who appear in To Kill a Mockingbird.