What do writers write about?

In the end, Glen Duncan reckons, the same things over and over again. It does not matter if you are writing a literary novel or a genre page-turner, the concerns repeat and repeat. A blessing or a curse.

The end in question here, by the way, is the publication of By Blood We Live, the final book in Duncan's werewolf trilogy, a trilogy of monthly transformations, bloody violence, vampires, militant Christians and, umm, the poetry of Robert Browning. Yet, like the other two books in the series and the seven lit-fic novels Duncan wrote before, it is also a book about love and sex and cruelty and death and the existential absence of meaning, "all the stuff I'd been writing about from the beginning," he laughs.

Duncan is an Anglo-Indian Roman Catholic boy from Bolton who is now a London novelist trailing critical acclaim and, thanks to the werewolves, some commercial success. The latter was, he admits, what he was hoping for when he started writing The Last Werewolf. It is the story of Jacob Marlowe, a 200-year-old man who once a month turns into a beast and eats someone, and, as the title suggests, the last of his kind (or so he believes...). In the writing, though, it morphed into something a little more than intended.

"To be totally honest, I tried my best to write what could be described as a page-turner; very heavy on plot, lots of action, lots of cliffhangers, but not overburdened with literary anxiety or metaphysical questions. But for better or worse, once I had started with a 200-year-old protagonist who had spent most of his time reading and becoming a Scotch connoisseur with this magical immunity to lung cancer and STDs, thematically it was business as usual."

The result - as is the case for both of its sequels too - is a page-turner with heft. A smart, fast yet thoughtful read. There are plenty of plot twists if that is what you are after, but much of the fun is in the way it applies literary tricks to genre tropes. That and the bone-crunching and skin-tingling physicality of the writing.

"We have all seen werewolf transformations hundreds of times on screen," Duncan says. "We have grown up in an age where there is nothing that cannot now, courtesy of computer-generated imagery, be convincingly rendered in the visual field. But what the movies can't do is give you the interiority of the experience. So what I wanted to do was have all the stock moments in the werewolf myth - the transformation, the kill and so on - but to really do it from the inside, to try and get inside the skin and imagine what that might feel like rather than what it would look like."

Why werewolves though? Partly that was a commercial choice too. We were not short of vampire books after all, he points out. But more than that it was the idea of the wolf that scared him as a boy. "Werewolves were far more terrifying than vampires. It is probably the idea of seeing the human within the beast and knowing you can't reach it. It might as well be a great white shark. There is no sitting down and discussing Proust with it, which the traditional vampire model seems to leave room for. You can have a conversation. It was the notion of a reasoning being somehow trapped in a form that left it impervious to pleas for mercy or appeals to reason that was particularly nightmarish to me."

The boy he was grew up as the only non-white child at both his primary and secondary schools. "One of the benefits - obviously it was not all fun as you can imagine - was it did marginalise me in a fruitful way, in that I found myself often on the periphery of the social world looking on and tried to figure out how it all worked. So I did cultivate an observational disposition."

Would he have swapped that for being part of the pack though? "I probably would have at the time, but obviously in hindsight it was one of the great blessings. I am not sure, to be honest, if I had grown up without that slight sense of alienation that I would have become a writer at all."

And yet, he says, he also grew up in a house of storytellers. His father, in particular, would tell him stories of his Indian homeland, an "almost imaginary world" to Duncan, all of which helped validate the idea of storytelling as something one could aspire to. Maybe it was that habit that set him on his current path.

But he can pinpoint the moment when the reader decided to become a writer almost exactly. "I read John Irving's novel The World According To Garp when I was about 14 or 15. It was the first grown-up book that I had read. It is the story of a young man who grows up to be a novelist. I finished it and I wanted to write a book that made the reader feel the way I felt at the end of that, which was sort of both bereft and elated."

In his previous novels Duncan has shown an interest in the extremes of human behaviour. You can find sadomasochism (Weathercock), the war on terror (A Day And A Night And A Day) and pornography (Hope). But a werewolf inevitably puts itself outside culture, outside society, outside morality, of course. Eating people will do that. But what, all three books ask, if you eat people and there is no comeback for the act. Lightning does not strike. God does not speak. What does that mean for the meaning of life?

"Intellectually or rationally," Duncan says, "I don't see that there is any evidence to suppose that life is meaningful or that the universe was created with a purpose or any of that stuff. But at the experiential level, given that we have got an imagination and a sophisticated consciousness, one that seeks patterns, I think we are just stuck with that traffic jam of deep-down feeling it is meaningless and yet fairly frequently experiencing it as meaningful."

Writing a novel is a search for meaning in itself, you might say. "It is. It is a paradox because any novel that is formally worth its salt is a bit of a lie because life does not have that shape. By and large all fiction is an imposition of a structure. But we are doomed to tell stories and are doomed to love them."

And maybe as one of the vampires says in By Blood We Live (yes, there are a few, and though none of them quotes Proust, Browning, as previously mentioned, does get a look in), novels do something more than just tell stories. The more you read them the harder it is to condemn human behaviour.

"That is my sense of the relationship with literature and the arts in general, I suppose. It is the corrective attempt to accommodate imaginatively - which is not the same necessarily as condoning ethically - everything human. Good fiction enlarges our imagination, enlarges our empathy and enlarges the room. We have to understand things, whether we object to them or not."

And that's a place to start from.

By Blood We Live, by Glen Duncan, is published on Thursday by Canongate , £7.99