Theroux will speak at the Hampstead and Highgate Literary Festival about his new work The Secret Books, which tells the story of Nicolas Notovitch. Or one version of the story...

Marcel Theroux takes out two hardback books and plonks them on the table. One is sleek and blue, the other brown, battered and looks like it has passed through many hands.

“This was the seed, this mysterious looking volume,” he says, turning it over in his hands. “I’d been interested, sort of obsessed with it for about 10 years.”

The book he holds is The Unknown Life of Jesus Christ, published in 1894 by Nicolas Notovitch, a Crimean Jewish writer, adventurer, spy and, well, who really knows?

“I was always interested in Jesus’ missing years,” he continues. “In the Bible, there’s this weird gap in between when he’s 13 and about 30; he just disappears from the Bible and there’s lots of apocryphal tales of what he might have been doing including coming to Cornwall on some sort of tin boat.”

Theroux’s latest book is an exploration into another theory: Jesus spent these years studying Buddhism in a Tibetan monastery, a theory supposedly confirmed by Notovitch, who stumbled across the “proof” when he found himself recovering from a broken leg in India. Notovitch’s book, written originally in French, was subsequently widely discredited.

The Secret Books – his sixth novel – is about the nature of stories and the act of telling them. It’s about “narratives” and alternative histories, what could have been the case if we had looked at something differently, “the other histories that don’t get accepted, that we don’t read, as maybe there are other versions than the one we’re stuck with”.

“The big idea is that the world we inhabit is a story we tell,” he says. “I don’t think that the world is only a story we tell. I think there are facts and things that happened and there are things that are beyond question, but I think the world we inhabit is a story. People pick selectively and cherry pick facts that suit their story. We all do it.”

The book begins with him at the end of his tether, unable to start a novel, when he encounters a person at Tooting Lido who sells rather bizarre office equipment. In the shop, he comes across some wax cylinders on which an elderly Notovitch had recorded the events of his life.

“I thought about writing a historical novel: you know, ‘I am Nicolas Notovitch,’ or ‘early one morning Nicholas Notovitch got up and made himself a cup of tea…’ and I was in a kind of despair because it seemed remote and irrelevant and at best a kind of competent fakery to write a standard historical novel in that way, so, for better or worse, the solution I came up with was to dramatise my own struggle.

“I envisioned it like one of those movies where they say, ‘it was 1887 and I was on my way to…’ and it dissolves into a scene and suddenly you’re in the middle of it.

“Obviously I nicked that,” he adds, giving credit to The Princess Bride.

Telling the story in this way allows Theroux to jump in and out, cross-examining and picking up on certain nuggets. Like the use of an unreliable narrator, this method gives space for the reader to consider for themselves just how much of the relayed information is trustworthy, and whether or not this actually matters as much as the story itself.

“I don’t want to say that Nicolas Notovitch was a liar,” he says. “I want to at least explore the possibility that he was telling the truth because apart from anything it would be really nice! But it gave me room for him to both be lying and telling the truth.”

The facts that Notovitch was Russian and that Theroux has expertise in Russian affairs (“I’m a Russian-ist…is that a word?”) turn out to be coincidental, but this did allow him to draw in a lifetime of study and research and his career as a journalist.

In his other life making documentaries (for the BBC and Channel 4, including Unreported World), he spends a great deal of time involved with Russia, meaning he could read Notovitch’s numerous letters, and accumulated a huge amount of information, but decided to keep most of that out of the book “because, on some level, you have an obligation to entertain your reader”.

“I did think maybe this is a biography,” Theroux says, when asked whether this could have been a work of non-fiction.

“But in the end I was more interested in projecting myself into his mind. What on earth was he doing? I had a theory about what he as really up to and it wasn’t because Jesus was a Buddhist. It was far more to do with Russian history of the period and latent anti-Semitism.”

One of the other books to which “The Secret Books” refer, he says, is The Protocols of the Elders of Zion, which purports to be the minutes of a meeting held by a Jewish council at which they discuss plans for world domination: “the mother of all fake news stories and the model for all subsequent conspiracy theories.”

“Here’s the thing, it’s such an obvious forgery, it’s been plagiarised from another book, there’s no way it can be true and yet it still has an afterlife. David Duke is still peddling the Protocols on his website.”

The former Grand Wizard of the Klu Klux Klan uses the Protocols to “prove” that Jews are planning to take over the world, and it has been used in the same way ever since it was written, despite the fact that it is a work of fiction.

As Theroux writes: “Anyone who cares to examine it can find out the truth. But the thing about a work like this one is that those who believe don’t care about the facts. It doesn’t have to be true, because it feels true.”

Theroux says he never set out to write a book about anti-Semitism, and admits to being unaware until recently of the extent of anti-Semitism in Russia, but when he noticed the coicidence of the Protocols with Notovitch’s life in Paris, he couldn’t ignore it.

“A book can’t kill people,” he says. “Human beings kill people, but the Protocols was an inspiration for pogroms and Hitler talked about it in Mein Kampf, and what was fascinating about it was it was forged in Paris, around about the time that Notovitch was coming back from India with his gospel.”

There was never an intent to make his novel timely, but Theroux can’t deny that there are relavancies with the insurgence of far-right and neo-Nazi groups into the mainstream.

“If the book does nothing else,” Theroux says, “I hope it makes people notice that book, because it’s in our water supply whether we like it or not; people are imbibing it. When people talk about conspiracy theories, they are talking about this book.

“And the fact that in 2017 there’s a bloke a couple of steps removed from the guy who sits in the White House who thinks it’s true…I mean, it’s insane. It’s a really scary calumny.”

Marcel Theroux will be discussing The Secret Books (Faber&Faber, £12.99) at the Hampstead and Highgate Literary Festival at 6pm on Sunday September 17. Tickets £10: jw3.org.uk