How do you find the strength to carry on after losing your husband and only child within 20 months of each other? If you’re Joan Didion, you “maintain momentum.” You try to heed your much-missed daughter’s sanguine philosophy and resist “dwelling on it.” You throw yourself into book tours for “The Year of Magical Thinking,” the searing memoir you wrote in the insanity of grief over your husband’s sudden death. You immerse yourself in your first Broadway show, based on that book. And then you push yourself some more, to write about the dying of the light.

It’s been six years since Didion’s adopted daughter, Quintana Roo Dunne Michael, died at age 39 of complications from virulent flu, pneumonia and a cerebral hemorrhage after nearly two harrowing years in a series of ICUs. Didion opens “Blue Nights,” her book about losing Quintana, on what would have been the seventh anniversary of her daughter’s 2003 marriage to Gerry Michael at the Cathedral of St. John the Divine in New York City. She recalls the wedding in evocative detail, including the white stephanotis woven into Quintana’s hair, her red-soled Christian Louboutin shoes and the fancy peach-colored cake. Didion comments: “Her choices, all. Sentimental choices, things she remembered. I remembered them too.”

Blue Nights” is a devastating companion volume to “Magical Thinking,” a beautiful condolence note to humanity about some of the painful realities of the human condition that deserves to be printed on traditional black-bordered mourning stationery. It loosely braids two strands: salient memories of Quintana’s short life, and Didion’s somber, belated recognition of “the end of promise, the dwindling of the days, the inevitability of the fading, the dying of the brightness” in her own life. The book takes its title from the gorgeous long blue twilights that precede and follow the summer solstice, what the French call “l’heure bleue” and the English call “the gloaming.” Didion explains, “Blue nights are the opposite of the dying of the brightness, but they are also its warning.”

Although Didion says that writing “no longer comes easily,” she summons her signature spare, plainspoken prose and assertive two- or three-word paragraphs to powerful effect. (“Time passes.” “I said that.”) She also relies, sometimes to a fault, on an almost incantatory use of structural repetition: “There had been cars. . . . There had been agapanthus. . . . There had been English chintzes.” As if shuffling the clues for a fresh take on the insoluble riddle of how Quintana’s story might have had a different ending, she returns repeatedly to the same few scenes from her daughter’s unusual childhood. These include the unexpected call from a Santa Monica doctor hours after Quintana’s birth in 1966 saying he had a beautiful baby girl for Didion and her husband, writer John Gregory Dunne, to adopt. This family lore became an unwitting source of anxiety for Quintana, who worried what might have happened had they not answered the call.

Didion evokes young Quintana with a series of charming images: Quintana designating a projection room in her dollhouse, Quintana learning at an early age to differentiate between “on expenses” and “not on expenses” and ordering triple lamb chops and Shirley Temples at fancy hotels while on location with her screenwriting parents. She is less precise — in fact, often frustratingly elliptical — about her daughter’s mental and physical illnesses. Alluding to “the startling depths and shallows of her expressions, the quicksilver changes of mood” later diagnosed as borderline personality disorder, she asks, “How could I have missed what was so clearly there to be seen?”

‘Blue Nights’ by Joan Didion (Knopf)

Didion’s main subject, however, is not the tragedy of Quintana’s curtailed life, but of Didion’s current sorry state. Her self-portrait is unsparing. She confesses that she was an ill-prepared mother, and wonders whether her expectations forced Quintana to try to grow up too soon. Like so many working mothers, she feels guilt at the memory of Quintana’s list of “Mom’s Sayings” posted on her garage clubhouse: “Brush your teeth, brush your hair, shush I’m working.” Didion asks painfully, “Was I the problem? Was I always the problem?”

In the aftermath of Quintana’s death, Didion’s increased fragility and mounting medical problems lead to a long-avoided confrontation with “the certainties of aging, illness, death.” One of the saddest passages in this heartbreaking book involves Didion’s difficulty “sitting in frigid waiting rooms trying to think of the name and telephone number of the person I want notified in case of emergency.”

The marvel of “Blue Nights” is that its 76-year-old, matchstick-frail author has found the strength to articulate her deepest fears — which are fears we can all relate to. It’s not death she’s afraid of but “what is still to be lost” — including the ability to keep memories of her daughter within close reach at all times.

Heller McAlpin , a frequent contributor to Book World, reviews books for NPR.org, the Christian Science Monitor, Barnes & Noble Review and other publications.

BLUE NIGHTS

By Joan Didion

Knopf. 188 pp. $25