How did a story about the discovery of evil views in a revered parent turn into a universal parable about the loss of innocence — both the inevitable loss of innocence that children experience in becoming aware of the complexities of grown-up life and a cruel world’s destruction of innocence (symbolized by the mockingbird and represented by Tom Robinson and the reclusive outsider Boo Radley)?

The depiction of Atticus in “Watchman” makes for disturbing reading, and for “Mockingbird” fans, it’s especially disorienting. Scout is shocked to find, during her trip home, that her beloved father, who taught her everything she knows about fairness and compassion, has been affiliating with raving anti-integration, anti-black crazies, and the reader shares her horror and confusion. How could the saintly Atticus — described early in the book in much the same terms as he is in “Mockingbird” — suddenly emerge as a bigot? Suggestions about changing times and the polarizing effects of the civil rights movement seem insufficient when it comes to explaining such a radical change, and the reader, like Scout, cannot help feeling baffled and distressed.

Though it lacks the lyricism of “Mockingbird,” the portions of “Watchman” dealing with Scout’s childhood and her adult romance with Henry capture the daily rhythms of life in a small town and are peppered with portraits of minor characters whose circumscribed lives can feel like Barbara Pym salted with some down-home American humor. And it reminds us that “Mockingbird,” the novel, was more concerned with the day-to-day texture of Scout and Jem’s lives and the world of Maycomb than “Mockingbird,” the movie, which focused more closely on Atticus and Tom Robinson’s trial.

The advice Ms. Lee received from her first editor was shrewd: to move the story back 20 years to Scout’s childhood, expanding what are flashbacks in “Watchman,” used to underscore the disillusionment Jean Louise feels with the present-day Atticus, now 72. (“I’ll never believe a word you say to me again. I despise you and everything you stand for.”) Scout’s disillusionment in “Watchman” oddly parallels that of Jem in “Mockingbird,” after Atticus fails to get Tom Robinson acquitted, and Jem realizes that justice does not always prevail.

Another pivotal difference between the two books concerns the decision to make Scout (“juvenile desperado, hell-raiser extraordinary”) the narrator of “Mockingbird” — a decision Ms. Lee executed with remarkable skill, managing the stereoscopic feat of capturing both the point of view of a forthright, wicked-smart girl (who is almost 6 when “Mockingbird” begins) and the retrospective wisdom of an adult.

Somewhere along the way, the overarching impulse behind the writing also seems to have changed. “Watchman” reads as if it were fueled by the alienation a native daughter — who, like Ms. Lee, moved away from small-town Alabama to New York City — might feel upon returning home. It seems to want to document the worst in Maycomb in terms of racial and class prejudice, the people’s enmity and hypocrisy and small-mindedness. At times, it also alarmingly suggests that the civil rights movement roiled things up, making people who “used to trust each other” now “watch each other like hawks.”

“Mockingbird,” in contrast, represents a determined effort to see both the bad and the good in small-town life, the hatred and the humanity; it presents an idealized father-daughter relationship (which a relative in “Watchman” suggests has kept Jean Louise from fully becoming her own person) and views the past not as something lost but as a treasured memory. In a 1963 interview, Ms. Lee, who now lives in her old hometown, Monroeville, Ala., said of “Mockingbird”: “The book is not an indictment so much as a plea for something, a reminder to people at home.”

One of the emotional through-lines in both “Mockingbird” and “Watchman” is a plea for empathy — as Atticus puts it in “Mockingbird” to Scout: “You never really understand a person until you consider things from his point of view.” The difference is that “Mockingbird” suggested that we should have compassion for outsiders like Boo and Tom Robinson, while “Watchman” asks us to have understanding for a bigot named Atticus.