Joan Didion's The Year of Magical Thinking was a book that seemed to lend a certain credibility to a form  the memoir  that is sometimes regarded with suspicion, as a kind of sabbatical from the writerly duty to be interested in things other than oneself. On the contrary: The Year of Magical Thinking was instantly identified as a necessary book. An account of the year that followed the sudden death of Didion's husband of 40 years, the screenwriter John Gregory Dunne, it articulated a private experience  the experience of bereavement  for which it is difficult to find public expression. Blue Nights is a tragic kind of sequel to The Year of Magical Thinking, describing the subsequent death of Didion's daughter Quintana, aged only 39: where the earlier book drew its strength from the common well of suffering, the follow-up must grapple with the awful particularity of Didion's misfortunes.

Some of the best literary memoirs  Lorna Sage's Bad Blood, Mary McCarthy's Memories of a Catholic Girlhood, Paula Fox's Borrowed Finery  describe the author's survival of extreme or extraordinary circumstances. Others The Year of Magical Thinking is one  document the effects on the author of events (in this case the death of a spouse) which, though difficult to bear, are universal and ordinary. The former might be said to be turning chaos into order, transforming or redeeming chaotic experience with the orderliness of the author's prose and the rational mind from which it issues; the latter begin with the proposition of order and proceed, through the honesty of their writing, to challenge and disrupt it. This is a delicate and difficult undertaking, for obvious reasons. To personalise common experiences is to assert a version of them with which others might not agree. The memoirist, while placing an unusual degree of trust in the reader, is also exposing herself to their judgment. She hopes to speak for everyone; she risks being ridiculed for speaking only for herself. Yet the very thing she is often exposing  as in Didion's case  is her vulnerability.

The memoir, being nominally a confessional form, has an interesting relationship to honesty; much like the confessional box itself, the question of whether disclosure is a need or a duty is obscured somewhere in the covenant between penitent and priest. The penitent is tired of harbouring her sins; she wishes to invoke the judgment that will free her from the truth she has told. But will she prize this freedom quite so highly if instead of forgiveness, damnation is the consequence of her honesty? The penitent wants to disburden herself, to get things off her chest. The memoir-writer may at the time not be sure why he or she wishes to place private material in the public domain, but perhaps  if "experience" can take the place of "sin"  it's for much the same reason.

The Year of Magical Thinking, published in 2005, was honest about numerous things: what it's like when someone you know and love dies right in front of you, what it's like to come home alone from the hospital and scrape the dinner you'd prepared for you both uneaten off the plates, what grief feels like, what trauma feels like. Didion's candour about these things was welcome, for in bereavement the public and private struggle to be reconciled: between mourning as public fact and mourning as private experience lies a gulf that Didion, as a writer, was able to navigate. The delineation of this uncharted space  between public and private, between what seems and what is  defines the moral core of memoir as a form. The book also had a strong streak of narcissism on which the reader, like the priest, was called to exercise forgiveness. "I wore a short white silk dress I had bought at Ransohoff's in San Francisco on the day John Kennedy was killed," Didion wrote, as a random instance, recalling her wedding day. The glamorising of her own experience, what might be called the sin of self-importance, risked undoing her achievement by alienating the "ordinary" reader on whose recognition and empathy the memoirist depends.

In The Year of Magical Thinking, the countless references to fame and social privilege have a second passport, as it were: they are an indelible aspect of the life Didion lived with her husband, and by enumerating them with such repetitive insistence she was also, in a sense, cataloguing the effects of her 40-year marriage. In Blue Nights they are, again, omnipresent, but the effect is altogether more troubling. Quintana died shortly after her own wedding and the death of her father, a series of events whose cruelty Didion has barely, by her own admission, managed to survive. "This book is called Blue Nights because at the time I began it I found my mind turning increasingly to illness, to the end of promise, the dwindling of the days, the inevitability of the fading, the dying of the brightness." These are sentiments with which the reader can only sympathise, yet the question of what kind of book can be made from them remains. The universality that was the basis of The Year of Magical Thinking no longer applies: losing a spouse is a common aspect of human experience; losing a child is not. The death of a child is unmitigated chaos: which writer could ever hope to exact order from it?

Didion's strategy, or rather her instinct  the instinctive response to chaos  is to repeat herself. She struggles to revive the form and style of her earlier book, to make it live again; she repeats anecdotes, and often sentences, word for word; she creates repeating prose patterns whose effect, in the end, is to confer the author's own numbness on the reader. What she cannot do is master her own material: instead of grieving with her, we are watching her grieve. This is a piteous and exposing process, and one which places a moral burden on the reader. And it is here that Didion's lack of humility comes back to haunt her, for by burdening the reader she is also making herself vulnerable to judgment. Early on, describing a set of photographs of Quintana as a child, she writes: "In a few she is wearing a cashmere turtleneck sweater I brought her from London when we went that May to do promotion for the European release of The Panic in Needle Park." What passed in The Year of Magical Thinking as the camaraderie of husband and wife becomes, at a stroke, something more disturbing  a kind of parental attention-seeking that again and again drives Didion's sentences away from their subject and back to herself. "Was I the problem?" she asks. "Was I always the problem?"

Blue Nights does contain an element of authenticity: Didion takes her time getting to it, but once there she is somewhat freed from the compulsive repetition and self-reference in which she has until now become increasingly entangled. It is the story of Quintana's birth family (she was adopted as a baby) and its members' attempts to contact her in adult life. Quintana's parents, after they gave her up for adoption, got married and had two further children. One of these children  Quintana's sister  tracked her down using the internet and a private detective, and Quintana, the privileged only child, was suddenly availed of a whole extended family in Dallas. "In Dallas these strangers had shown her snapshots, remarked on her resemblance to one or another cousin or aunt or grandparent, seemingly taken for granted that she had chosen by her presence to be one of them." Quintana struggled to cope with these revelations, and wrote to her birth mother and sister saying she needed time to adjust to them. "In reply she received a letter from her mother saying she did not want to be a burden and so had disconnected her telephone." When Quintana died, Didion notes drily, "her sister sent flowers".

This brief flaring of Didion's writerly power has its corollary in her admission of her own increasing physical and emotional fragility. Blue Nights is in a sense the manifestation of this fragility, the dwindling and fading of the artist's ability to create order out of the randomness and chaos of experience. Didion's portion of this chaos has been cruelly and unjustly large: it comes as no surprise that in the end, she should not be able to digest it all.

 Rachel Cusk's The Last Supper is published by Faber.

"I realise as I write this that I do not want to finish this account." So wrote Joan Didion in the final pages of her memoir The Year of Magical Thinking. And no one who accompanied her on that raw journey of grief and loss could blame her. The memoir was completed in 88 days, less than a year after her husband, John Gregory Dunne, died from a heart attack. If ever a piece of writing sprang from an impulse to claw a loved one back from death's grasp, it was this one. And if "finishing" meant letting go in some crucial and final way, then what sane person would want to finish?

But, as any reader also knew, another equally tragic story remained "unfinished". As Didion's memoir about Dunne drew to a close, their daughter Quintana, 39, and just married, was seriously ill with pneumonia and septic shock. By the time the book came out the following year, she was dead. You could not read Didion's memoir without knowing this and it coloured everything: how could one person endure so much tragedy in so short a space of time? Blue Nights, an account of Quintana's life and death, is an attempt perhaps to answer this terrible question. It stands as a shocking companion to the first book.

Didion and her husband adopted Quintana Roo  named after a territory they spotted on a map of Mexico  as a baby. "Once she was born," says Didion, echoing the feelings of all parents everywhere, "I was never not afraid." Yet she worries that this fear transmitted itself to her daughter, for Quintana grew up an anxious little girl, gravely, sometimes comically, precocious for her years. Aged only five, she rang a local asylum to find out "what she needed to do if she was going crazy". Another time she called 20th Century Fox "to find out what she needed to do to be a star".

But given the life Quintana led  accompanying her glamorous parents all over the world to movie premieres and publishing houses ("When does she get the money?" she asked Didion's agent, having sat in solemn silence throughout the meeting)  this precocity doesn't seem at all surprising. What is a little surprising is Didion's almost wilfully fuzzy understanding of it.

She takes Quintana to a "harrowing" film and reports bemusedly that she chose simply to ape the "default response" of adults around her and say "I think it's going to be a big hit". It turns out that the film was Nicholas and Alexandra, and Quintana was "four or five" years old. I saw that film as a teenager and am still haunted by the blood dripping down the walls after the Romanovs are executed. Could any parent really consider it suitable viewing for a five-year-old?

I admit that it was around this point that I started to lose my initial inhibitions about criticising Didion (like so many others, I felt changed by reading Magical Thinking and fully expected to feel the same about this book). But it was time to go with what my head was telling me: that, though I felt for its creator, too many things in this narrative were making me uneasy.

The fact, for instance, that a whole section is devoted to the death of Natasha Richardson. The Redgrave/Richardsons and the Didion/Dunnes were, and are, close friends, and Quintana and "Tasha" as good as grew up together, but I still can't see what these pages add except to confirm what we all know: that tragedy comes in random, often accidental forms. Even the repeated phrase "This was never supposed to happen to her" has a dead-end emptiness that finally starts to irk.

Meanwhile, the least charitable part of me couldn't help noticing that the deaths were piling up. There's the close friend who collapses with a cerebral bleed  Didion visits her in the intensive care unit with a mutual friend called Lenny. And then: "The next time Lenny and I were in the ICU at Cedars together it was to see her and Nick's daughter Dominique, who had been strangled outside her house in Hollywood." Didion's conclusion that "It occurred to me as I walked home that I had seen too many people for the last time in one or another ICU" leaves an odd taste in the mouth.

But then this is a very odd book, full of fury and fragility and yet somehow anaemic. In fact, Didion's heartfelt declaration that "there is no day in her life on which I do not see her" serves only to remind you of Quintana's essential absence. Because we, the readers, do not ever really "see" this girl. Even the passages where she might have come to life are rendered needlessly brittle by Didion's stabbing, birdlike prose with its constant repetitions and exhortations such as "Do note..." and "Let me repeat..." until, again, that uncharitable part of you wants to grab her by the sleeve and say, "Just don't!"

Where the book is most successful  and most poignant  is in the viciously honest picture Didion draws of a lonely, encroaching old age. Underweight, refusing to eat (Didion tells us again and again that she's always been thin, insists she's eating the tub of Maison du Chocolat vanilla ice-cream in her refrigerator, and yet it's very clear that friends are worrying about her weight), your heart breaks for her increasing and incurable frailty, both physical and emotional.

"You have your wonderful memories," people say to comfort her but, as she astutely and chillingly observes, memories provide no solace. Didion doesn't want memories, she wants the real thing  her husband and her child  back with her. You feel that when they went, they took her life with them. And the line that finally succeeded in breaking my heart was actually the simplest, the plainest, said of Quintana: "I need her with me." Yes, I thought, you do. She does.