There are very few fiction writers who seem, wonderfully, to transcend the problem of how to write, or of "literary style". Their prose sounds, we feel, utterly natural, transparent, as the world would sound if it could narrate itself to us. Naturally, the paradox is that it takes supreme artistry to achieve that feeling of artlessness, of presenting life "as it is". Tolstoy and Chekhov are the dual masters; these days critics deservedly cite the Canadian short story writer Alice Munro. Jhumpa Lahiri grazes the same heights in her new collection, Unaccustomed Earth.

That, however, is not the only surprise about this book, the third from its feted, 40-year-old American-Bengali author, who won the Pulitzer Prize for her 1999 debut, The Interpreter of Maladies. The new collection is an unexpected popular sensation in her native United States, entering the New York Times bestseller list at number one in April after a healthy dose of pre-publication hype. Lahiri's US publishers, Knopf, clearly guessed there was something eminently saleable about these finely observed stories of loss and love set amid Bengali immigrant families, and they were right. But the real news lies elsewhere: Unaccustomed Earth contains some of the best, most beautiful fiction written this decade - the kind that will be read 50 years from now.

The five stories collected in part one turn a penetrating eye on parents and children who have arrived in suburban America - often Massachusetts - from Bengal. Lahiri is a dedicated miniaturist; these stories are unfailingly domestic in their scope, understated in their tone. In the quietly devastating "Hell-Heaven", for example, a daughter remembers her mother's unspoken love - unnoticed at the time - for a male house guest, who later marries an American woman. In "A Choice of Accommodations" Amit, husband and father, attends the wedding of his school crush Pam Bowden and feels that his best years, already, have slipped behind him. In "Nobody's Business" an awkward college PhD student, Paul, falls in love with Sang, his Bengali-American housemate.

Of course, much has been written about what is called the "immigrant experience" over the past ten years. What is important, here, is that Lahiri's work is the literary opposite of the fashionably sprawling and noisy fiction that has often resulted. And the difference originates in her compressed, miraculous, acutely subtle prose.

That prose is at its sublime best in the title story, in which Ruma - once a successful lawyer, now a stay-at-home mom in suburban Seattle - and her father move around each other uneasily after the death of Ruma's mother. This is writing that relies on a deeply Chekhovian kind of unflashy quietism, and which does its work by accretion, not sudden, spectacular jolts. That means its beauty can be glimpsed only obliquely in the way that Ruma's father, flying to Seattle, "stared out of the window at a shelf of clouds that was like miles and miles of densely packed snow one could walk across". Or the way Ruma watches "a vast breeze work its way through the treetops, a grander version of the way Akash used to sigh when he was a baby".

It is thanks to this deft instrument, however, that Lahiri can depict, superbly, the gentle way that Ruma and her father misunderstand each other. Ruma fears that, in the aftermath of her mother's death, she should ask her father to live with her; but her father feels liberated by the death of his wife and wants to continue touring Europe with a new female friend, Mrs Bagchi.

The untraversable distance between parent and child - and the way it is made wider still by arrival in a new country - is a seam that Lahiri mines repeatedly and with brilliance in Unaccustomed Earth. Her American-Bengali children are perpetually translating contemporary America for their bewildered parents, and so perpetually caught between living their lives and only observing them.

Part two, entitled "Hema and Kaushik", gives us three more narratives, but interlinked so that together they form a loose-knit novella. We follow our two protagonists from childhood - 12-year-old Hema develops a crush on Kaushik when his family stays with hers on arrival in Massachusetts - through to their incomplete, unsatisfying and separate adult selves: Kaushik becomes a photojournalist, chasing death in war zones, and Hema an academic who has spent 11 years orbiting a married man.

Across 120 pages, Lahiri shows just how deep she can reach with her carefully transparent prose, which - and this is the crucial part - seems to license her characters to develop, free from any authorial intention or identifying stamp. As with all the stories here, an all-pervading sense of loss courses through "Hema and Kaushik"; it is a loss symbolised by immigration, but which, in fact, we all must experience: the leaving behind of the blank slate that is youth - of infinite openness and possibility - and arrival at the compromise and limitation that is adulthood.

Hema and Kaushik eventually meet again and realise their long-buried need for one another. The last few pages of the book amount to a spare, unexpected, devastating crescendo, moving but utterly without sentimentality. They tell us everything we need to know about Lahiri's awesome literary talent.