She grabbed her children and ran with her husband behind the hotel and into a waiting jeep. They were driving away when the wave hit.

Deraniyagala suffered an internal injury as the jeep turned over, and when she regained consciousness she was spinning around and around, bleeding, naked from the waist down, covered with mud, and with her mouth full of sand. She was quite alone. There was no sign of the rest of her family. For a while there was a strange calm: "I was floating on my back. A flock of storks was flying above me."

Then she saw a child floating towards her, a boy, clinging to the broken seat of a car. For a moment she thought it could be one of her sons, but she realised as the child was swept past that she was mistaken: "I stared into this unknown landscape, still wondering if I was dreaming, but fearing, almost knowing, I was not."

The most moving passages in this extraordinarily moving book are in the section that follows: Deraniyagala is rescued and it becomes clearer with every passing minute that she is the only survivor of her entire family. With bleak clarity she recalls her thoughts as the realisation dawns on her that her entire world has come to an end. The restraint of her prose, with its short, simple declarative sentences, makes the scale of the horror all the more unbearable: "Someone suggested I take a sleeping pill. I refused the pill. If I sleep now I will forget. I will forget what happened. I will wake believing everything is fine. I will reach for Steve, I will wait for my boys. Then I will remember. And that will be too awful. That I must not risk."

As she is taken to her aunt's house in Colombo, she writes of her quiet determination to end her life, huddling beneath the covers of her cousin's bed, hoarding sleeping pills, stabbing herself with a butter knife and "smashing my head on the sharp corner of the wooden headboard of the bed". She surfs the internet to find painless ways of killing herself. "I needed to know how to do it successfully, I couldn't mess it up."

For months, anything to do with children - toys, a cricket ball, the sound of small running feet - brings back the horror in overwhelming floods of grief. There are periods of pure madness. When her brother sells her Colombo house, she stalks the Dutch family that has moved in: "Strangers in our home. It's ghastly. The Dutch family, settling in there like nothing has happened. They must be dancing around in their f***ing clogs."

She drives there night after night, repeatedly ringing the doorbell at 2am, blaring the horn and blasting out her husband's favourite Smiths songs on the car stereo: "Asleep are we? Not for long, you won't be."

Slowly, she finds the strength to carry on. On one occasion she drags a friend on a crazy drunken expedition to see a place where turtles come ashore to lay their eggs. "It's Friday night," the friend barks eventually. "I could be in London, I could be down the pub. What am I doing on a godforsaken beach looking up a turtle's arse."

She visits and revisits the site of the hotel where she was staying when the wave struck. Once, her father-in-law finds fluttering at his feet a laminated A4 page from the research report her husband was writing when he was killed. "Except for a small tear in the middle, this page was intact. It had survived the wave? And the monsoon in the months after? I clasped the paper to my chest and sobbed."

She finally stops her search when she finds her son's shirt "under a spiky bush, half-buried in sand. One of the sleeves was still rolled up."

It is two years before she can bear to return to the family home in London. Even then she can't live there, but after finding her husband's eyelash on a pillow and a dish with the remains of a beef curry he had cooked, she decides instead to keep it as a shrine, unchanged in any way from how it was when the family set off to fly to Sri Lanka. Seven years later, "their absence has expanded … Seven years on, it is distilled, my loss."

The scale of her tragedy is so overwhelming that "it still seems far-fetched, my story, even to me. Everyone vanishing in an instant, my spinning out from that mud, what is this, some kind of myth?"

This is possibly the most moving book I have ever read about grief, but it is also a very, very fine book about love. For grief is the black hole that is left in our lives when we lose someone irreplaceable - a child, a parent, a lover. It is the negative image that, in its blackness, reveals love with a greater clarity than its positive counterpart.

A grief like that suffered by Deraniyagala can have no resolution, but all books must end, and this one does on a pitch-perfect note of elegiac sadness and something close to acceptance, as she sits in anonymous exile in New York, "like I'm in a witness protection scheme", remembering happier times: the moment her son called her not by her name but by that of her husband - "Mummy Lissenburgh" - as if she had "no identity without these three boys … Steve enjoyed our son's account of me, of course. He egged him on. 'Clever boy, Mal, spot on, you're exactly right. You tell it like it is.' So, 'Mummy Lissenburgh!' Malli would chant. And the three silly boys would fall about laughing. Now I sit in this garden in New York, and I hear them, jubilant, gleeful on our lawn."