The heat in Maggie O'Farrell's new novel is uncomfortable but not unbearable.

It is 1976 in London, gardens are shrivelling during the hosepipe ban, and when recently retired Robert Riordan walks out the door without explanation and disappears, family secrets start to seem uncontainable. It is one of the inevitabilities of O'Farrell's novels that what is repressed will rise to the surface, that there are sleeping monsters in the form of things untold and long-nurtured misunderstandings.

As if by way of explanation for all that happens, O'Farrell tells us in one of her framings: "Strange weather brings out strange behaviour ... As a Bunsen burner applied to a crucible will bring about an exchange of electrons, the division of some compounds and the unification of others, so a heatwave will act upon people. It lays them bare, it wears down their guard. They start behaving not unusually, but unguardedly. They act not so much out of character, but deep within it."

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O'Farrell is adept at creating pace out of the intricacies of family relations. She keeps the gas up on the Bunsen burner throughout. The flame, a readerly appetite for revelation and domestic reconciliation, is almost at times too hot, too driving, making it hard to pause and linger on her more contemplative passages. I felt that I gobbled this book.

The story glides along, shifting effortlessly from shoulder to shoulder of the key family members: Irish mammy Gretta and her children, Michael Francis (the eldest), Aoife (the rebel) and Monica (the good girl, who has always assumed she is favourite). It is careful with divulging its secrets, and one of O'Farrell's skills is knowing how to tell enough, yet not give it all away. She captures, too, the way families so often are in crisis. Almost everyone seems to be aware that there are more important things to be getting on with, but at the same time irresistibly preoccupied with small talk and bickerings.

In fact the more sensational secrets are not really the driving force of the narrative. Why Robert Riordan disappears is the mystery that sets the story in motion, and an Irish Catholic culture of shame is what has kept things unspoken, but O'Farrell appears more interested in something more universal: the lifelong psychological impacts of how we arrive in the world and how we become parents. At the heart of this novel, as with her last, The Hand That First Held Mine (which won the Costa award), is a difficult birth and a struggle with the swamping task of early parenting.

Aoife, Gretta's youngest daughter, we learn, came into the world too fast and inconveniently, and in the months that followed "howled incessantly", driving her mother to tranquillisers. Her first months in the world were not just difficult for Gretta, but also for Monica, who saw her mother turn into a "ghost-like wisp of a woman", and longed for her to return "to being the way she used to be, before all this: large and in love with life, always popping in and out of other people's houses or jamming on her hat to go and see the priest".

O'Farrell, like Rachel Cusk who has created similar explorations of the family, is particularly deft at describing the struggles, triumphs and losses of motherhood. Notably eloquent are her depictions of Claire, Michael Francis's wife, as seen through her husband's eyes. Up until recently, we learn, she moved around almost always with one or two of her children attached. "He never saw her outline," O'Farrell writes. "She had become like one of those matryoshka dolls with the long-lashed eyes and the swirl-painted hair, always containing smaller versions of herself."

We see the mourning of that intimacy further down the line in Gretta, whose house is littered still with the memories of her small children. "The air, for Gretta, still rings with their cries, their squabbles, their triumphs, their small griefs. She cannot believe that time of life is over. For her, it all happened and is still happening and will happen for ever. The very bricks, mortar and plaster of this house are saturated with the lives of her three children. She cannot believe they have gone. And that they are back."

Gretta, who wears giant floral smocks, pops pills and likes to talk so much that when her husband disappears her temples ache from stories "not spoken, unlistened to", is perhaps O'Farrell's most compelling creation. "Why did she have to be so overweight, so eccentrically dressed, so loud, so uninhibited, so wild-haired, so keen to tell everyone her life story?" ponders Michael Francis.

A fabulous, often comic figure, she seems to sum up the beauties and flaws of an old Irish-Catholic way of living. O'Farrell does not indulge in a great deal of cultural analysis but, when she does do so, it's here in this affectionate if also critical depiction of a generation who fled poverty in their homeland. But the beauty of O'Farrell's writing is not in its historical detail. Nor is it in any Jonathan Franzen-style skewering of our own generation's foibles. What she does is burrow into the hot, unchanging depths of family. She uses the bright light of her Bunsen burner to lay family bare, to make us ponder its dark hiding places. And it is decidedly moreish.

Instructions For A Heatwave

Maggie O'Farrell

Tinder Press, £18.99