Stefan Hertmans is an elder statesman of Flemish literature – a poet, novelist, short story writer and academic. Only a handful of his books have been translated into English, including the novel War and Turpentine, which was longlisted for the Man Booker International prize in 2017. If, like me – to my shame – you know little about Flemish literature, The Ascent would be a pretty good place to start. It is a crash-course in 20th-century Flemish history and culture. Prepare to descend.

Based on years of research and augmented with personal reflections and fictional episodes and illustrated with rather elliptical and mournful black-and-white photographs, the book tells the true story of Willem Verhulst, a Flemish nationalist from Ghent who became a “confidential agent”, a Vertrauensmann or V-Mann, or what Hertmans simply calls “a Jew-hunter for the Waffen-SS”. It turns out that Hertmans had ended up buying the beautiful old Verhulst family home, “for a sum that would not even buy me a mid-size car today”.

Hertmans brilliantly describes and imagines scenes in the house. He interviews various surviving Verhulst family members, and takes particular issue with the account of Willem’s life in a book by his son, a famous Flemish professor of history, Adriaan Verhulst. So, it’s a story about a family, a primer on the history of collaboration with the Nazis during the war, and a book about a house. But at the centre of it all is Willem Verhulst.

Willem is a sad, pathetic creature, almost too good – that is, too awful – to be true. Blind in one eye – a “Flemish cyclops”, according to Hertmans – Willem comes across in his quoted letters and diaries as a weak, vain and foolish man, though also half-conscious of his many sins. “In any case […] what I should have seen, I did not see; but I also saw many things I’d have been better off not seeing, and pretending I hadn’t seen a thing was often useful in later life, a habit that was difficult to break.” He becomes an enthusiastic Nazi, founding something called the Comité voor Dietsche Actie, whose grand aim is the unity of all Dutch-speaking peoples in a fascist Greater Netherlands. In reality, most of his time is spent filing reports to send Jews, communists and Boy Scout leaders to their deaths.

It is a pitiless portrait. But Hertmans’s greatest disgust and dismay is reserved for Willem’s son, Professor Adriaan Verhulst, who in his research for his book about his father seems not to have bothered to view the records of his trial, which detailed his various crimes. “Nothing was stopping him from facing up to the truth, but he chose not to,” writes Hertmans. “If he’d decided otherwise, his portrait of his father might have been less forgiving.” Hertmans does all the unforgiving on Adriaan’s behalf. Discussing the fate of an employee of Willem’s, who was part of the resistance then mysteriously disappeared, betrayed by someone, Hertmans remarks, “The conclusion must have presented itself to Adri – an emeritus professor of history, hunched over his humiliating manuscript, the son of a ‘bad’ father – but he did not write it down.”

Hertmans writes it all down. There are memorable novelistic flourishes throughout the book as he imagines family life in the home that became his own. “In every respect, Willem has transformed into a new man. Sometimes Mientje feels a warm glow when she sees how contented and self-confident her husband has become; but sometimes she lies awake at night wondering why he hasn’t come home, what he’s up to this time, until, close to dawn, she hears the soft scrape of the heavy front door across the tiles as it opens and shuts, and she falls into a heavy, gloomy sleep, waking up with a start an hour later because she has to get the children out of bed.” Most memorable though is the account of one of the Verhulst daughters, Letta, whose job it was to deal with the rats which infested the house, drowning them in a water barrel in the back yard. “The squeaking still rings in my ears […] whenever I dared to open my eyes for a second.”

The Ascent is published by Harvill Secker at £16.99. To order your copy for £14.99 call 0844 871 1514 or visit Telegraph Books