Ferdinand Mount’s outrageously fascinating quest-story, Kiss Myself Goodbye, is a tale of family secrets, money and lies. At its heart are the glamorous uncle and aunt of the author, a British political commentator, and the unanswered questions around their histories. How much, for example, did uncle Greig Mount know about the source of his wealth, or the life of his tirelessly social, luxury-loving and — we come to discover — spectacularly untruthful wife? Perhaps a lot more than he cared to admit to his poorer, and more proper, relatives.

Unca and Munca, the names by which young “Ferdy” was taught to address the high-living couple to whom he became a sort of adopted son, derive from The Tale of Two Bad Mice, one of Beatrix Potter’s most subversive works. After vandalising the doll’s house they are supposed to guard, Potter’s mice, Hunca Munca and Tom Thumb, slip a crooked sixpence into the dolls’ stockings, to “pay” for the damage. This fictional deception — on a larger scale and in real life — turns out to resemble what the terrifying Munca and her compliant spouse are engaged in doing. Everybody, including their baffled nephew, has been hoodwinked. Some in their path (the couple’s manipulative treatment of their adopted daughter, Georgie, provides one of the book’s most harrowing chapters) will be destroyed.

Told like a detective story, Mount pursues the truth about his aunt as layers of deceptions peel away, artfully concealing until his final pages the ultimate, jaw-dropping clue to Munca’s wealth. An enthralling memoirist, Mount opens with a portrait of his prim and puzzled younger self striving for a clear view of the evasive Munca (definitely much older than she claims), the mystery of her well-hidden “brother” Buster (in fact, Munca’s seven-times-married rogue of a son), and of the source of wealth for a life she leads with gleeful opulence.

Rags-to-riches was seldom more extremely contrasted than here. Munca’s hidden life begins in the backstreets of Victorian Sheffield, an industrial hellhole that nearly robbed both Engels and Orwell of adequate invective. At one moment, Munca and her sister find themselves living on the streets. The next — or so it feels — Munca retains a permanent suite at Claridges from which to entertain an appreciative young Ferdy to suppers at The Ivy.

Along the way, sharing his bemusement at each new revelation, Mount concocts a wonderfully skewed history of the 20th century. “Brightside Revisited” (Mount adores wordplay) returns us to the Sheffield in which Munca’s supposed father died aged 45 of Grinder’s Lung, a lethal ailment caused by labouring in steelworks. A brighter theme emerges from Munca’s son’s love-affair with motor-racing (Buster changed cars faster than he did his wives). I loved Mount’s accounts of early racing cars buzzing around Brooklands’ concrete walls “like flies” — and flying off the track like “huge beetles”: an oddly exact image.

Most surprisingly of all, having tracked Munca and her second husband to a gloomy mansion block in Marylebone, Mount discovers that TS Eliot and his first wife live in the flat upstairs. Munca herself was no reader, but she did share Vivienne Eliot’s mania for dancing; Mount makes the notion of his aunt doing a tango with “Tom” appear not just beguiling, but probable. Where Munca was concerned, as her nephew clearly shows, nothing ever exceeded possibility. And nothing and nobody — including, poignantly, her own poor Georgie — was ever going to stand in Munca’s way.

“Kiss Me Goodbye”, Munca’s favourite song, suits her insouciantly devastating progress perfectly. Using the song’s jaunty lyrics to close his book, Mount sounds understandably relieved to bid farewell to his aunt, having introduced his fortunate readers to one of contemporary literature’s great monsters.

Kiss Myself Goodbye: The Many Lives of Aunt Munca, by Ferdinand Mount, Bloomsbury, RRP£20, 272 pages