THE poet Czeslaw Milosz once said: “When a writer is born into a family, the family is finished.” But what if that family already contains a writer? In such cases, one can imagine youth as a long and fruitful apprenticeship. This haphazard training programme has produced some quality material. One wonders how some writers might have turned out under different tutelage. What would Auberon Waugh have been without Evelyn, or Martin Amis without Kingsley? Who knows. What is certain is that, however hard they try, writers of this kind can never shrug off their childhood. Their origins are always easy manna for journalists hunting for copy.

Marcel Theroux might have presented numerous documentaries, published six novels – his latest, The Secret Books, is a tour de force – and written travel articles for years, but he is still used to being “the other Theroux”. His father is the great travel writer Paul Theroux, and his brother, Louis, is a household name on account of his televised Weird Weekends. I meet the amiable and laidback Marcel Theroux – tall, well-groomed, wearing jeans and a light blue shirt – on a bright shining day at the Edinburgh International Book Festival. Naturally, we start chatting about his family. Does it bother him that people cannot talk about him without reference to his father and brother?

By way of answering, he tells me a story. Earlier this year he made a documentary called Putin’s Family Values, about how the Orthodox Church are propagating a reinvigorated conservatism that is shaping modern Russia. A few weeks before it aired, Theroux was in Vietnam, in a bubble tea shop, when someone from the Radio Times rang him up. The interviewer said: “Listen, you sound like Louis and you look a bit like Louis so how come you aren’t as successful as Louis?"

"I was thinking," he says, "why aren’t you as successful as Louis? I don’t even see myself in the same game.” He is in the same game as his father, however. But there is little family rivalry and the elder Theroux frequently gives him notes on his travel writing. Growing up, the young Theroux understood that “books had an importance in our house that they didn’t have in other houses. But it meant there was no mystique around writing”.

As the above encounter highlights, Theroux is worldly in person and in print; he lives in London, but he was born in Uganda and spent two years in Singapore while growing up. At one point, he studied international relations at Yale (English literature was more to his taste). A constant traveller, he wrote The Secret Books on the road, writing out scenes and testing paragraphs longhand in three large notebooks (he was in Jaipur when he was “on the home straight”). His novel’s fictional landscape is similarly global. It moves from 19th-century Kerch, a village on the shores of the Black Sea, through Tsarist Russia to revolutionary Paris and back east to a Tibetan monastery, as it follows the life of Nicholas Notovitch.

Notovitch was a real man. As proof and as a useful curio, Theroux has brought an old, leather-bound copy of Notovitch’s The Life Of Saint Issa, the heretical gospel originally published in 1894 that purports to document Jesus’s lost years between the ages of 12 and 33. It sits on the table between us in our small white marquee, the kind you might find used as a field hospital in some far-flung war-torn country. It was 2002 when Theroux became interested in Jesus’s life story. “I thought there was a possible documentary to be made about Jesus in India or if Jesus was a Buddhist. It is an interesting current in Christianity. You are presented with the authorised Bible, but there is a whole load of apocryphal texts that aren’t in the Bible, and there are other influences on Christianity that we don’t even talk about.”

Then Theroux discovered Notovitch’s book. “I thought, flipping hell, what an amazing Indiana Jones-type story, that a Russian journalist was rooting around in a Tibetan monastery in 1887 and found a manuscript that showed Jesus studied Buddhism in his lost years.” Notovitch’s book has, understandably, been treated with scorn and is widely viewed as a hoax. The journalist in Theroux could not resist travelling to the monastery in Ladhak to speak to the vice-abbot. “A part of me continues to hope that it is true. The novel always presents you with a possibility that it is true.” This possibility is delineated through Theroux’s careful use of framing devices. The central one has the narrator finding a 1933 recording Notovitch made of his life story and adumbrating it for the reader.

“I really wrestled with how to tell the story. At one point, I did start writing it as a conventional historical novel. Not quite, ‘it was a dark and stormy night as Nicholas Notovitch rode up to the monastery’, but you can imagine what it might have been like. There was something dismaying about it. I thought, I want to summon him up and ask him what happened – to give him the best possible chance to state his case.” Theroux doesn’t take this truth-telling too seriously. That is partly why the novel is pitted with hilarious anachronisms, such as 19th-century characters discussing Pussy Riot and Darth Vader; it sounds as if it shouldn’t work, but it does, constantly testing our faith in what we are reading.

Holding Notovitch’s book – which is also a kind of memoir – I am reminded of how many other books are contained within The Secret Books. I open the frail pages but only cast a cursory glance through them. They are written in French, which was no problem for Theroux, who can also speak Russian. Russia has been a lifelong interest. His previous novel, Strange Bodies, is in part about the Bolshevik God Builders who thought they could use science to realise their dream of immortality. “The more I’ve gone to Russia the more I’ve been fascinated by it. Russia’s history is like a microcosm of all the fascinations that have gripped the world in the 20th century, but taken to the extreme …the Russian Empire was a huge part of his [Notovitch’s] makeup, and I thought, this is my way in. When I found out he was Jewish it deepened my understanding of Russian history because I didn’t understand how deep Russian anti-Semitism goes, and what a constitutive part of Russian history it is, what influence Russia had over the foundation of the state of Israel, and how it incubated Russian anti-Semitism.”

In The Secret Books, Notovitch’s life is intimately bound up with Russian spy and arch political manipulator Rachkovsky, who runs Russia’s secret service headquarters in Paris. Rachkovsky blames Jews for all the disorder in the world and his work culminates in the publication of the notorious anti-Semitic tract, The Protocols Of The Elders Of Zion. From this point, the novel becomes a battle of the books. One of the central revelations of the secret gospel is that it exonerates the Jews of any guilt for Jesus’s death. Notovitch, recording his story in 1933, is all too aware of how history can be turned against an entire people. That one of the central tenets of Christianity is called into question gives him hope that the world might listen to his truer, less hate-filled story. But Rachkovsky knows that people need to hate, regardless of the truth. As he says about his sinister publication, “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.”

All this talk of rewriting history is a reminder that outside our small tent writers are busy peddling their own faith in stories. “Everyone says how wonderful and generous storytelling is, but stories are other things as well. Stories are propaganda and poison and manipulation,” says Theroux.

If history is the clash of competing narratives, the 20th century could be described as a victory for propaganda over truth. He didn’t set out to write about something so grand, and neither did he set out to write about anti-Semitism or the ruthless duplicity of politicians. After completing The Secret Books, however, he realised it held “unintentional echoes” at a time of fake news and when racial hatred is on the rise. (“Rachkovsky could have been in Trump’s cabinet, don’t you think?”) Theroux now understands that anti-Semitism is “the hatred that never dies” and the “shape of other prejudices”. But he is not surprised at what he learnt on his journey to finished novel. After all, where humanity is concerned, “there’s nothing new under the sun”.

The Secret Books is published by Faber & Faber, £12.99