In the preface to her 1968 essay collection “Slouching Towards Bethlehem,” Joan Didion included a line that, in just six words, encapsulates the strange business of writing nonfiction: “Writers,” she explained, “are always selling somebody out.”

In the years since, those six little words have transformed the act of writing into a delicate ethical transaction between writer and subject, predator and prey. The catch, of course, is that writers are “selling somebody out” even when they admire their subject and intend to celebrate them on the page. The crime, as it were, is that the writer forces the subject—an ordinary person, someone unaccustomed to being written about or interviewed—to see himself in a light far different than the one in which he sees himself. As Janet Malcolm would later write in “The Journalist and the Murderer,” her 1990 reflection on Didion’s six words, writers prey “on people’s vanity, ignorance or loneliness, gaining their trust and betraying them without remorse.”

In that sense, one might say that Joan Didion has made a living selling people out. Her essays, after all, are often ruthless in their honesty: take, for instance, her 1976 profile of James Pike, the progressive Anglican bishop who was an early advocate of racial desegregation and LGBT equality. Didion concluded that Pike represented nothing but the most superficial dimension of American spiritual life. His “sense that the world can be reinvented smells of the Sixties in this country, those years when no one at all seemed to have any memory or mooring,” she wrote, “and in a way the Sixties were the years for which James Albert Pike was born.”

In recent years, however, Didion—whose clean, supple prose and sharp insights into the paradoxes of countercultural America have earned her a much-deserved seat among the greatest essayists this country has produced—has shifted her focus.

Although famous for inserting personal reflection into her journalistic essays, her last three books have been, more or less, memoirs—or, rather, the type of memoirs that only Joan Didion could write, which move between memory, analysis, commentary, and sometimes cultural history. “Where I Was From,” released in 2003, is a meditation on her native California that presents the history of her family through the history of the state. 2005’s “The Year of Magical Thinking,” which won the National Book Award and made her a celebrity, chronicles the year in which she lost her husband and lifelong companion, the writer John Gregory Dunne. And “Blue Nights,” out today, comes to terms with the death of her beloved daughter, Quintana Roo Dunne, at the age of 39.

Each of these contains a far more personal dimension than usually appears in her essays and her novels. And yet “Blue Nights,” in which Didion addresses her life as a mother, the heartbreak of her daughter’s death, and, finally, her own mortality, has a sense of finality the others do not. In a sense, it is her most forthcoming and most intimate book yet. The question, then, is what to make of a book in which a writer with so much experience “selling somebody out” essentially does the same to herself. To her credit, Didion is as honest with her readers about Joan Didion as she has been about all her previous subjects.

The book draws its title from the “blue nights” of early summer at northern latitudes, where, as Didion writes, “the end of summer is fast and lost in the blaze of the dropping sun.” These nights, she maintains, signal a change of season, the coming of summer, and, simultaneously, a slight suggestion of the darkness to follow. “Blue nights,” she writes, “are the opposite of the dying of the brightness, but they are also its warning.” Regarding her time with Quintana, Didion admits that she believed these blue nights would never end and that the darkness would never come.

It did, of course, come. Over Christmas 2003, Quintana contracted the flu. She developed pneumonia and septic shock and then, on December 30, her father died of a heart attack while Didion made a salad: “You sit down to dinner and life as you know it ends,” she wrote in “The Year of Magical Thinking.” After a brief recovery, Quintana suffered a cerebral hemorrhage in the spring of 2004. After a year of hospitalization, she died the following summer.

The honesty of “Blue Nights,” of course, comes from the way Didion relates her own experience as an adoring mother. As attentive as she was to her only child, adopted in 1966, Didion confesses her failure to recognize Quintana’s anxiety and fear of abandonment. “I had seen her wishing for death as she lay on the floor of her sitting room in Brentwood Park,” Didion writes. “Let me just be in the ground, she had kept sobbing. Let me just be in the ground and go to sleep.” As a memoirist, Didion resists the temptation to make excuses for her shortcomings as a mother, to obfuscate the truth, or to have the last word. In every situation she describes, the fault, if anyone’s, is her own. “She was already a person,” she writes of Quintana. “I could never afford to see that.” At no point in “Blue Nights” does the blame fall on anyone other than herself.

Didion is even honest about what is probably the most common criticism of her later work, what the critic John Lahr once dubbed the “Brentwood Blues”––her anxiety and uncertainty over own her own values and her position in a world of privilege. After all, she mentions such details as Quintana wearing Louboutin heels to her wedding, that the cake she served came from Payard, and that the first time she tried caviar occurred at Chicago’s storied Ambassador Hotel. Didion makes no apology for the charmed life she and shared with her husband and Quintana, but she nevertheless addresses the point: “I wanted to lay this on the table,” she says, and, with similar candor, there is no other subject she refuses to discuss.

That no subject, in a sense, is banned from Didion’s table is ultimately what gives “Blue Nights” its power. While she confesses that after Quintana’s death she “was no longer, if I had ever been, afraid to die,” what is even more remarkable is that she is not afraid to tell.

—Staff writer James K. McAuley can be reached at mcauley@fas.harvard.edu.