During the heatwave of summer 1976 a devoted husband and father of three gets up from the breakfast table and goes out to buy a newspaper. He doesn't come back. Robert Riordan was recently retired, but still there was nothing to suggest to his wife Gretta that he was unhappy or about to do a disappearing act. Gretta is adamant that she has no idea where he is or why he has gone. Robert and Gretta's grown-up children descend upon the family home to scratch their heads and console their mother.

The matriarch of a complicated Irish family who settled in London, Gretta is an unreconstructed 1970s character and all the more entertaining for that. She's a hypochondriac and a religious fanatic. Her children are her obsession but also the bane of her life. And now they're all here under their feet, united in grief and confusion about the fate of their father. Even though they've come together in a disaster  or is it?  Gretta can't help feeling smug that at least all her children are around her.

The portraits O'Farrell paints of the three siblings here are painfully believable. The only son, Michael Francis, is a history teacher whose marriage is failing. His wife has "rediscovered" herself and taken up with a group of intellectual new friends. He knows it's at least partially his own fault but he's getting left behind and is not sure what to do.

Monica, the older sister, also has a difficult marriage, to an older man. Her stepdaughters are barely speaking to her and blame her for the death of the cat. She seems almost to be living a fantasy where she's expecting someone to rescue her. There's a sense that life has left her behind and she had been promised better things. Her neurosis is real but it's also (intentionally) comical.

Aoife, the youngest, has always been the black sheep of the family and now, in her early 20s, has banished herself to New York where no one will have to think about her and worry (of course, they do anyway). She and Monica have not spoken properly for years after an incident no one can bear to mention. Aoife is plagued by her own deficiencies: she clearly has undiagnosed dyslexia and is functionally illiterate. Which means she probably should not have taken the job as an administrator for a photographer whose invoices she cannot read.

O'Farrell doesn't shy await from complex dynamics, and despite juggling four separate narrative threads here (the disappearance of the father plus the back stories of the three siblings), things never become confused. The writing is always intense, perfectly crafted and just right. The characters authentically drawn. This allows her to get away with a lot.

So what's not quite right? There's a strange sense of lack of urgency over Robert's disappearance, and he comes across as not only missing from the family but also missing from the novel. Maybe that's deliberate, and it certainly adds to the mystery. (Plus, we get an inkling pretty early on that Gretta actually has a good idea of where he is.) And the heatwave, which initially provides the necessary oppressive feel, suddenly seems to recede into the background never to return. But perhaps that's better than the protagonists sweating and taking illegal baths on every other page.

You care very little about these contextual details, though, when everything else is so expertly done. This is O'Farrell's sixth novel, and in the space of those half a dozen books she has managed to cement herself as a reliable storyteller and a master of the family drama. Instructions for a Heatwave builds on that consistency.

All the hallmarks of an O'Farrell novel are here: a family with secrets in its past and words left unsaid years ago, relatives long since forgotten, a claustrophobic atmosphere of uncomfortable emotional closeness. This is an accomplished and addictive story told with real humanity, warmth and infectious love for the characters. Highly recommended.

Maggie O'Farrell never fails to deliver, yet her dependable brand of eagleeyed storytelling rarely strays into the formulaic. Following her Costa-winning The Hand that First Held Mine, this sixth novel is classic O'Farrell, a tale of family relationships that uncovers layer after layer of secrecy and misunderstanding.

The novel opens in the summer of 1976 which, as its title suggests, experienced a heatwave of legendary proportions. In the third month of the drought that stifles London, encrusting it with red-backed aphids and fuelling acts of insanity, Robert Riordan goes out to get his morning paper, as he has done for over 30 years, while Gretta Riordan bakes bread and the couple's three adult children pursue seemingly ordinary lives elsewhere. But today, Robert is home later than usual. Gretta "calls her husband's name, once, twice. The flank of the garden wall throws the sound back to her." He has disappeared.

This act instantly dismantles a flimsy family structure balanced between rebellion and conformity. Gretta has "done her best to keep Ireland alive in her London-born children", providing Irish dancing classes, regular church attendance and holidays to Connemara, yet has bred Londoners with minds and grievances of their own. The oldest, Michael Francis, is the one who never got away. As a student, precocious fatherhood and domesticity have disposed of his dreams ("I was going to do my PhD, sleep with everyone I could lay my hands on, then go to America" ). He teaches at a grammar school and rents a flat near his childhood home.

At this pivotal point, chaos has descended: his children are newly neglected in favour of his wife's Open University degree; she leaves the house in squalor, and is busy finding feminism and herself while her husband stumbles through the dinosaur mists of patriarchy. This is a painful portrait of a marriage whose failure reflects the politics of an era, as well as a wincingly true depiction of reality in conflict with fantasy. O'Farrell skilfully portrays women's lives in the 1970s, poised between oppression and progress, and the prejudice experienced by the Irish in England.

In the meantime, the middle Riordan, Monica, is suffering her own rural hell in the company of hostile stepdaughters after a broken marriage. Having lost a child, Monica is flailing. Strangely, though both her personality and dilemma are affecting, there is a chill to her character  her siblings are far more appealing.

We journey into the past, with Gretta unexpectedly pregnant with her third baby: the lifelong source of worry that is Aoife. A child of the 1950s, the illiteracy that is Aoife's "own private truth" is, of course, undiagnosed dyslexia, and O'Farrell's descriptions of words jumping and sliding away are disturbingly effective. In adulthood, Aoife ups and leaves the family home for New York, where work and romance are blighted by her condition.

These are ordinary lives acutely observed, with a brilliant dissection of different generations' attitudes towards the same predicament. Only Robert's disappearance can bring the Riordans scurrying back home  and it is here that the pace slackens. As the protagonists wilt in the heat, limply theorising about what might have happened, the same lassitude and claustrophobia set in for the reader. The Riordans wait for something, and so do we. But O'Farrell is too skilful a writer to lose her readers: she cuts to the chase in the nick of time, re-routing the quest to Ireland where she picks up slack threads for an ending that is revelatory, redemptive and moving.

"Strange weather brings out strange behaviour", and what could be a glib construct in a lesser writer is so well handled that it serves to strengthen the novel's structure. This is essentially a series of character studies and an examination of family dynamics, but with a plot sufficiently substantial to carry it. As ever with O'Farrell, it is her manipulation of time that infuses the most humdrum of stories with brilliance and suspense. Decades are travelled seamlessly, sometimes mid-sentence, without ever obfuscating.

Here is an author whose depth and insight hover just below the surface of an apparently effortless lightness. She can capture "a slight wrinkle in the atmosphere" in passing, the popular and the profound almost disarmingly intermingled. There is a deliciousness to this novel, a warmth and readability that render it unputdownable and will surely make it a hit. She's done it again.

 Joanna Briscoe's latest novel is You (Bloomsbury).