Ray Bradbury once wrote a story called All Summer In A Day.

It was set in a primary school on the planet Venus, where it's been raining non-stop for seven years. The children are too young to remember the sun, except for one girl who has recently arrived from Earth, and feels its absence more acutely than they do.

On the day it is due to come out again, if only for a couple of hours, the girl stands apart from the others, waiting for the sun and wanting to go home. The rest of the class hate her for that apartness. They lock her in a cupboard just before the clouds break, and then they all go out to play.

Junot Diaz first read that story when he was about the same age as the kids in it. His family had moved to the US from the Dominican Republic in 1974, when he was six years old. He was still struggling with English, but understood enough to identify with the girl in the cupboard.

He sat down at the back of a public library near his new home in New Jersey "and cried my little eyes out". In a short tribute published after Ray Bradbury's death earlier this year, Diaz wrote: "I never knew what I'd been experiencing as an immigrant, never had a language for it, until I read that story - It was my first real taste of the power of fiction."

His own tales of immigrant experience have not been nearly so allegorical. Drown, his first book of short stories, in 1996, tracked back and forth through the childhood and young manhood of a not entirely typical Dominican-American male – growing up in poverty in both countries, acting out racial and sexual attitudes acquired from both cultures.

That character, Yunior, went on to narrate Diaz's debut novel, The Brief Wondrous Life Of Oscar Wao, and most of the stories in his forthcoming collection, This Is How You Lose Her.

"The fascinating thing about Yunior," says Diaz, as if talking about a person he knows, rather than someone he invented, "is he has this completely bifocal point of view. He's got one eye on the most affluent country in the world, and the other aimed back at one of the poorest islands on the planet. The Catholic Church used to call it the power of bilocation – the ability to be in two places at once. Like Jesus."

Diaz makes this power sound not so much miraculous as super-heroic. "There are burdens that come with it, but there are also gifts." It is unclear, in conversation or fiction, to what extent Diaz is describing himself. Yunior's voice is commonly mistaken for that of the author.

When he first made it heard, with Drown, it sounded almost like a new language, accented with Spanish phraseology and inflected with street slang, explicit in some ways and reticent in others. "I had heart-leather like walruses got blubber," said Yunior in an early story called Boyfriend, talking up his own emotional toughness while exposing some of his weakness.

"You know what's funny?" asks Diaz today. "Yunior's voice is so artificial, so absolutely constructed. The more conversational and organic a mode of speech seems in prose, the more of an artefact it is. And because it is expected to be some natural extension of myself, it actually takes me 10 times more f***ing work."

On occasion, Diaz does sound a little like his fictional proxy. He has a tendency to blend emphatic profanity with higher-order academic word choices. Accounting for the 10 years it took him to follow those first stories, he describes the "excruciating exigency" of writing Oscar Wao.

The real trick, he says, "was keeping all my finger-biting, hair-pulling, face-bashing agony off the page". He measures his success by the apparent effortlessness of the finished novel – an exuberant and somehow joyous saga of Dominican immigrants haunted by the long shadow of the island's former murderous dictator, Rafael Trujillo. The book won the Pulitzer Prize for fiction in 2007, which doesn't seem to mean that much to Diaz: "It's a lot better than being ignored. But when the applause is over, and the ridicule, and the forgetting, and the world has moved on, are you still saying anything to anyone? Plenty of writers were hot s*** in their day, and now nobody gives a f*** about them."

This Is How You Lose Her took a further five years to complete, although some of the stories were begun more than a decade ago. "I always thought that I would write faster," says Diaz. "But the rhythm is the rhythm. I've learned that the universe is not on your f***ing schedule, and this s*** takes as long as it takes." The same applies to pain and loss, of course, and Yunior learns this too. The closing story of the new collection, The Cheater's Guide To Love, follows him into later life and pays him back in years of heartache for his earlier machismo.

Like Diaz himself, he's now a professor of writing at the Massachusetts Institute Of Technology in Boston – ethnically speaking, the whitest big city in the US. "The racism that Yunior encounters there is definitely representative. In the scale of diversity, I'd say that New Jersey is like London, and Boston is like the Orkney Islands."

Yunior's experiences are not necessarily Diaz's. In another new story, Invierno, the narrator remembers his first winter in New Jersey, the first time he saw snow. The author has many of the same memories, but his responses were different: "For one thing, I was a lot more scared than him. Immigration for a young kid is more terrifying than I've been able to portray. And there wasn't the same global awareness when we moved in 1974. There was no TV in our house in Santo Domingo, or even on our street. The only book I'd seen before was the goddamn Bible. I had zero images of the United States. I had no idea motherf*****s even spoke another language over there."

Decades later, Diaz's fiction contains information that might be instructive to white middle-class readers in the US and elsewhere. He'll concede that they're not getting it from other sources.

"This country is more diverse than it's ever been," he says. "With almost no limits. This s*** is wild now, man, and there's nothing on TV, in the movies, or the literature that comes close to describing it. American exported media is characterised by the profound unwillingness to deal with reality. But I believe that strength and value will arise from that reality, and not from some mystic formula where everybody looks and acts a certain way."

As a side note, Diaz mentions the increasing threat of deportation for non-citizens, and the fact that his own extended family "have never felt as unsafe in the US as we have in these Obama years". This is not a vote for Mitt Romney. "I assume that the Republicans would only be equally or even more hostile to immigrant communities than Obama has been."

He has never claimed to represent the whole Dominican diaspora. But he has said of his parents' generation's sufferings: "The fundamental by-product of trauma is silence." Today, he describes his writing as "pouring myself into those silences". Through Yunior and a few related characters, he talks about it because no-one else does.

Ask him who he's talking to, and he starts with his own younger brother. "He was born in New Jersey, so what the f*** does he know about what his parents went though? Not only does he not know, but a large part of him doesn't care. So you could say that my little brother stands in for everyone who is not familiar with this universe, and I'm as much in conversation with him as I am with anyone else."

That conversation, he admits, is a little one-sided. With the exception of one or two cousins, his family don't read his books, and are not exactly simpatico with his line of work. "They are profoundly militaristic," he says, listing off his father as an "army man", his sister who married a soldier, nephews who have served in Iraq and Afghanistan. His aforementioned muse of a little brother is an ex-marine combat veteran. "They look on my being an artist like a full-blown clinical derangement. They shake their heads and say, 'man, what happened to you?'."

It would be too neat for Diaz to answer "Ray Bradbury happened". As he puts it, Bradbury was only the first to help him realise "I just f***ing love books. I love to read. And when you love something, you want to be a part of it". He always thought he would write genre fiction, and is now at work on what he calls "an apocalyptic alien invasion virus zombie monster novel set in the Dominican Republic".

But even in his straight-up literary stories, Diaz has long since located an element of science-fiction in his own reality. "What is it to be an immigrant," he asks, "if not to exist in multiple worlds, simultaneously?"

This Is How You Lose Her will be published by Faber on September 6 (£12.99). Junot Diaz will appear with Nathan Englander at the Edinburgh International Book Festival on August 18. Visit www.edbookfest.co.uk or call 0845 373 5888