When does anticipation for a new book turn into something darker? For bestselling fantasy author George RR Martin, that point came when his fans – desperate for the next title in his hugely popular series A Song of Ice and Fire – launched an unprecedented flood of online abuse, berating him for everything from going on book tours to watching football games, just in case the author, self-described as "60 years old and fat", was to die before finishing his epic story.

"You don't want me to 'pull a Robert Jordan' on you," Martin blogged to the circling hordes of hungry readers, referring to a fantasy novelist who passed away before completing his magnum opus. "You don't want me doing anything except A Song of Ice and Fire. Ever. Well, maybe it's okay if I take a leak once in a while?" The resentment was such that fellow fantasy author Neil Gaiman was moved to come to his defence: Martin, Gaiman blogged, "is not your bitch".

So it was with a sigh of relief last month that Martin finally announced that A Dance with Dragons would be out this summer. Before that, impatient fans can feast on Game of Thrones, HBO's much-hyped new drama based on his series and named after its first novel, A Game of Thrones. Pitched as "Lord of the Rings meets The Sopranos", the 10-part show stars Sean Bean as the icily honourable northern patriarch Ned Stark, and looks set to make Martin even hotter property.

Though the author describes his books as "epic fantasy", A Song of Ice and Fire is no magic-and-maidens Tolkien rip-off. Dark and gritty, steeped in sex (some incestuous), it features minimal magic, maximum machiavellian machinations and, as favourite characters are variously beheaded, gored and poisoned, lashings of violence. Loosely based on the wars of the roses, centring around the drawn-out rivalry between the fictional Stark and Lannister clans, the series is set in a world where seasons last for years, and a colossal wall of ice protects the lands from an unknown evil beyond. The most recent novel in the series, 2005's A Feast for Crows, achieved the rare feat for a fantasy novel of reaching the top five of the New York Times bestseller list, suggesting that Martin's books are now attracting mainstream readers, too.

The author admits that there is still "plenty of snobbery" around fantasy. "Yes – 90% of fantasy is crap," he tells me. "And so is 90% of science fiction and 90% of mystery fiction and 90% of literary fiction. But I don't think the mere presence of an elf means it can't be a great book. Faulkner said that the only thing worth writing about is the human heart in conflict with itself, and I agree with that. But I think you can have the human heart in conflict with itself in a fantasy, in a mystery, in a romance novel – I don't think the genre matters."

Speaking from his home in Santa Fe, New Mexico, the extravagantly bearded Martin – the epitome of what a fantasy author ought to look like – sounds much happier today than he did when we met in person in November, when he was clearly still smarting about his fans' anger. After spending six years writing it, Martin has very nearly completed a book that is whopping even by his standards: over 1,600 pages in manuscript.

This unwieldiness is a familiar problem for Martin. The reason fans have been driven so wild over the six-year delay is because the fourth and fifth books were meant to be one. The sheer size of A Feast for Crows forced the author to split the narrative in two, meaning that some of the viewpoints of Martin's best characters – the scheming dwarf Tyrion Lannister, the dragon princess Daenerys – are missing, to be revealed in A Dance with Dragons.

"I've had fun at certain points [writing it], but perhaps less fun as it got late," says Martin, with some understatement. "Yes, it's wonderful to have thousands of people eagerly waiting for the book to come out. I just wish they'd be a little less vituperative when they write the emails."

As well as the satisfaction of finally getting "King Kong off my back", as he puts it, Martin is looking forward to seeing HBO's reimagining of his work. "They did great stuff with historical drama in Rome, with the western in Deadwood, the gangster thing in The Sopranos. They redefined each of these genres, took it to a new level. So they thought 'we could do that in fantasy, too'," he says. A former screenwiter, he'll be adapting one episode a season, with other writers tackling the remaining nine; this first series, he's taken on episode eight, a particularly bloody part of the action.

"Right now, it's more excitement I'm feeling, but I do have moments of 'Oh God, what if it's terrible, if it's a flop?' I worked out of Hollywood for 10 years" – on shows including The Twilight Zone as well as a handful of pilots that never saw the light of day – "and I had my heart broken half a dozen times, so I know all the things that can go wrong."

Born in Bayonne, New Jersey, Martin started writing A Song of Ice and Fire in 1991. He'd published short stories and novels since the early 70s before getting involved in television, but while he waited to see if a pilot and a film he'd written were going anywhere, he decided to go back to a science-fiction novel that he'd been pondering for years.

"I was writing away merrily on that, when suddenly the first chapter of A Game of Thrones came to me," he recalls. "So vividly I knew I had to write it. So I put the other book aside and I wrote that chapter, and by the time I'd finished that chapter I knew what the second chapter would be, and pretty soon the science-fiction novel was forgotten."

A 50-page cast list

His agent initially sold it as a trilogy: Martin had no idea it would eventually stretch to what will be seven books, thousands of pages and a cast list that, in its latest incarnation, tops 50 pages. He's no Tolkien, who had vast appendices and invented languages lurking behind The Lord of the Rings; these days it's getting harder and harder for him to keep track of his own creation.

"I have no special system," he admits. "I keep it in my head – or try my best to. Maybe I tossed a few too many balls in the air, but I have to keep juggling with them now." So will he try to rein himself in for the sixth and seventh novels? Possibly – though fans probably shouldn't hold their breath.

"Hopefully, the last two books will go a little quicker than this one has, but that doesn't mean they're going to be quick," says Martin. "Realistically, it's going to take me three years to finish the next one at a good pace. I hope it doesn't take me six years like this last one has. I have a million ideas. I have some other novels I want to write. I have a lot of short stories – I love the short story. But I've got to finish this first and then I'll decide what I'm inspired by at that point. If I'm not in some old folks' home." And if he is, no doubt his fans will be haranguing him even there.

• A Game of Thrones will be on Sky Atlantic on Monday at 9pm. For more on this series, read Saturday's Guardian Guide. A Dance with Dragons is published on 12 July by HarperCollins. Discuss this interview with Alison Flood today at theguardian.com/books

Five magic fantasy authors

Joe Abercrombie

Dark, bloody and very funny, young British author Abercrombie's writing features barbarians, torturers and conspiracies galore. No bespectacled teenage wizards here.

Gene Wolfe

The fantasy author for lovers of literary fiction. Try his multiple award-winning epic work The Book of the New Sun, set a million years in the future on a decaying Earth.

Stephen King

The horror master's move into fantasy with The Dark Tower series is gripping stuff.

Kelly Link

Criminally underrated, the fantasy short stories of this American writer (below) in Magic for Beginners and Pretty Monsters are creepy and wonderfully fun.

Guy Gavriel Kay

From the Arthurian-flavoured Fionavar trilogy to the medieval Provence of A Song for Arbonne, the Canadian author ticks all the right fantasy boxes. AF