Harper Lee, in an undated photo, avoided the limelight after the quick success of “To Kill a Mockingbird.” (Donald Uhrbrock/Time & Life Pictures/Getty Images)

The outcry over the recent shooting death of Florida teenager Trayvon Martin is another one of those eruptions in American life when you’d love nothing more than to hear what Harper Lee thinks. Or, more tantalizing, what she might write about it.

A similar longing informs “Hey, Boo: Harper Lee and ‘To Kill a Mockingbird,’ ” Mary McDonagh Murphy’s gently probing and completely engrossing documentary that saw limited release in theaters last year and is premiering (with a new title, “Harper Lee: Hey, Boo”) Monday night on PBS’s “American Masters” series.

It’s about Lee’s life and the immeasurable impact of her 1960 novel, “To Kill a Mockingbird,” but, really, it’s a film about how Lee will forever remain just out of our reach.

Now 85, the author is less of a recluse than a woman who just keeps to herself in her home town of Monroeville, Ala. Not long after the instant success of her novel about a miscarriage of justice in an Alabama courtroom — and its effect on a young girl and the small town she lived in — she turned away from the allure of New York-centric fame and chose a mode of lifelong contemplative retreat.

She spent the past five decades (and counting) with barely a published word to her name (a few essays in the 1960s, an open letter to Oprah Winfrey in 2006) and, despite the occasional anticipatory rumor, never released another novel. She just let it go.

“To Kill a Mockingbird” orbits in a literary stratosphere of permanence. Practically everyone who can read has read it. Most who have are in some way moved by the book or have had their opinion of racial equality shaped by it — or by the near-flawless 1962 movie adaptation of it. Even now, the Scout Finch in most of us will thumb through “To Kill a Mockingbird,” hoping to recover some sense of the topography that exists between right and wrong.

“Hey, Boo” is mainly a cultural study, bolstered by good interviews from a range of famous writers, thinkers and historians, but it also neatly balances the duty of biography with a look into some lingering mysteries about Lee and her work, absent an interview with the author herself. Lee hasn’t given a full interview in almost 50 years, not even to Winfrey, who took her to lunch at the Four Seasons in New York years ago and made her best (and nevertheless futile) pitch.

The film recounts Nelle Harper Lee’s upbringing, with its many parallels to “Mockingbird”: the lawyer father, who served in Alabama’s state legislature; the small-town life; the plain lessons apparent in everyday Southern race relations of the 20th century; the misfit neighbor who came to live with his aunt and, in one of American literature’s wildest coincidences, grew up to be Truman Capote.

Almost as good as an interview with Lee is an interview with her centenarian sister, Alice Finch Lee, whose voice has been reduced to a melodic gurgle but whose lucid insights into Harper’s motivations and post-“Mockingbird” life are telling. Another smart move the film makes is to let its sources — among them Andrew Young, James McBride, Anna Quindlen, Richard Russo, Allan Gurganus, Rick Bragg, Wally Lamb, Roseanne Cash, Tom Brokaw, Diane McWhorter and Lizzie Skurnick — talk at some length about and even deconstruct passages from the novel that still resonate.

With artful confidence, “Hey, Boo” organizes itself more around theme than chronology in a way that will surprise and entertain even those readers who think they’ve heard all there is to say on the subject of the novel, which began as a disorganized manuscript of short stories and character sketches and evolved, through years of intense revisions, into “To Kill a Mockingbird.”

Eventually, the film circles around to rumors, among them: Did Capote help polish Lee’s manuscript? “Hey, Boo’s” sources — friends of Harper — feel certain that Capote only ever saw the novel in typeset galleys and jotted a few comments, at best.

Lee and Capote’s long friendship, which included her travels to Kansas to help him navigate the reportage that would become his “nonfiction novel,” “In Cold Blood,” fell apart soon after “To Kill a Mockingbird” was released to huzzahs and won a Pulitzer. It’s easy to assume Capote was jealous of Lee, but “Hey, Boo” more charitably sketches it as two people racing away from each other in opposite directions: Truman toward fame, Harper away from it.

Why didn’t she write more? Assuming there are no finished manuscripts neatly arranged on a credenza awaiting posthumous publication, “Hey, Boo” offers some solace here, as its interviewees muse on the courage it takes to know when to quit. “She just wanted out,” her sister says. In “Mockingbird’s” autobiographical hues, it’s logical to look for the real Harper in the character of Scout. It might be more appropriate to find her in the guise of Boo Radley.

“Can you imagine the pressure?” Quindlen sympathizes, thinking of all the millions of people who yearned for Lee to write more. “I don’t know whether she couldn’t do it, but I prefer to think that she wouldn’t do it.”

‘Margaret Mitchell’

A far less fascinating “Margaret Mitchell: American Rebel” makes its television premiere just before the Harper Lee documentary Monday night, also under PBS’s “American Masters” rubric. The difference between the two films is a notable example of how much the quality can vary from one “American Masters” offering to the next.

While “Margaret Mitchell,” produced by Georgia Public Broadcasting, is also a biographical sketch of a perplexing 20th-century author who found immortal success with her one and only published novel (“Gone With the Wind”), the parallels mostly end there.

Given an hour, “Margaret Mitchell” and its interview subjects delicately gauge a mostly private life of contradictions and vague discontent — treading lightly and then moving on. Mitchell also struggled with her newfound celebrity and, in secret, made donations to educate black doctors in the 1940s. (To atone for racist feelings with which she was raised? It’s forever unclear.)

With its padded-out lingering over objects (typewriters, porches, roaring fires into which old manuscripts are tossed) and soft-focus reenactments, “Margaret Mitchell” plays like a DVD for sale in a museum souvenir shop.

Harper Lee: Hey, Boo

(90 minutes) airs Monday at 10 p.m. on WETA and MPT.

Margaret Mitchell: American Rebel

(one hour) airs Monday at 9 p.m. on WETA and MPT.