For school children across the planet, "To Kill a Mockingbird" has long been the literary prism through which to learn about civil rights and the struggle for race equality in America. The 1960 novel - and the 1962 film adaptation starring Gregory Peck - is, in part, a gripping tale of race politics in the segregated American South as six-year-old protagonist Scout (Jean Louise Finch) describes how her lawyer father Atticus tries to save a black man from a grave injustice.

"Go Set a Watchman" was the rejected manuscript from which Lee, on the advice of her publisher, refashioned a new novel that became "To Kill A Mockingbird." Watchman brings together the same key characters in the same fictional town of Maycomb, Alabama - only now it's the 1950s and we have moved on 20 years. It's the aftermath of the landmark 1954 Brown v. Board of Education case that ruled segregation in schools to be unconstitutional, a fractious time in the Deep South.

In Watchman, 72-year-old Atticus Finch opposes desegregation and attends Klu Klux Klan meetings. "Do you want Negroes by the carload in our schools and churches and theaters?" Atticus asks his 26-year-old daughter Jean Louise, who is visiting from New York and is shocked by her ageing father's racist views.

Meanwhile, Jean Louise's long-time admirer Henry Clinton, who works in Atticus' law firm - and who puts down his deep facial scars to a war-time brawl he had in a Berlin basement - shows a similar contempt for African-American rights.

No wonder that the millions of readers, and a cavalcade of reviewers, are registering shock as Watchman's details have gradually emerged in recent weeks. Sam Sacks of the "Wall Street Journal" called it a "distressing book" and a "startling rebuttal to the shining idealism of 'To Kill a Mockingbird.' This story is of the toppling of idols; its major theme is disillusion."

Revered as literature's great white civil rights activist, Atticus Finch has been recently been quoted in protests against the killing of unarmed African-Americans like Michael Brown - who was shot dead by a policeman in Ferguson, Missouri, in 2014. Could Watchman's uncomfortable truth shift Mockingbird's long-standing moral authority?

Should Watchman have been published at all?

Harper Lee is said to have feared publishing another book because Mockingbird, for which she won the Pulitzer Prize, was such a hard act to follow. The writer, who has rarely spoken publicly since the release of her first novel - she even rejected Oprah's request for an interview, explaining that "I already said everything I needed to say" and comparing herself to the reclusive Boo Radley character in Mockingbird - would not be surprised by the negative reaction to Watchman from lovers of her American classic.

Although Lee did not want a word changed of her original first draft of a long unpublished manuscript reviewers have been quick to compare it to a novel that underwent two-and-a-half years of editing and redrafting. Believed to be lost, Watchman was only rediscovered in 2014.

"The Telegraph's" Mick Brown opined that "perhaps it would have been a greater kindness to her reputation, and to the millions who cherish 'To Kill a Mockingbird,' not to have published it at all."

There was also a now discredited theory that an infirm Lee was unfairly pushed into consenting to Watchman's publication.

Written in the third person and not, as in Mockingbird, from Scout's engaging first person perspective, Watchman yet shows off Lee's eye for detail, her ability to squeeze so much life and nuance out of small town Alabama. And as "Time's" Daniel D'Addario writes, "Watchman is more successful as an amplification of characters it shares with Mockingbird, where they are better-developed." Since Atticus is no longer a saint but a conflicted and complex individual, Jean Louise is forced to reconcile her own beliefs.

Perhaps this novel, and not its illustrious predecessor, shines a more relevant light on race politics in America today. Atticus Finch, no longer the torch bearer for equality but a believer in white racial superiority, shows a deep vein of racism in the South that hit home in June when a white supremacist murdered nine African-Americans in a church in Charleston, South Carolina.

Lee's first novel, published so long after her second, is arguably the greater vindication of her moral position on race relations in the US.