Kiss Myself Goodbye. It sounds a bit like a William Boyd novel. It looks likea William Boyd novel, too: the cover shows an old hand-coloured photograph of a fur-stoled woman, determinedly leading a man in morning dress towards the camera. And, indeed, the raw material would likely make a very good William Boyd novel — only Boyd would have to jettison at least half of the breakneck hairpin bends in the mad, mazy plot for the sake of believability. This could be the ultimate case of a tale too strange to be fiction.

Ferdinand Mount has crafted the perfect, custom-made receptacle for his extraordinary story. The (anti)-hero is the millionairess Aunt Munca, self-named after the mouse in the Beatrix Potter story. It turns out to be just one of her many names and identities. Aunt Munca makes Moll Flanders look like a couch potato.

Mount opens with a brief memoir of his aunt and uncle as he remembers them from childhood. They are a glamorous, eccentric couple, constantly gliding up to London in the Rolls to take in a show and a night at The Pub (Claridge’s). Even then, there were questions and mysteries surrounding Munca. His uncle Greig (the blood relative, known as Unca) always seemed a lot younger than Munca. But younger by how much? Did Munca really grow up in the Philippines and ride her pony into the dining room for a dare? The young Mount doesn’t know much about her, but he’s pretty sure that she lies. A lot.

He also comes to realise in his early twenties how unhappy his cousin Georgie is. She is Unca and Munca’s only child. Munca has always been at pains to insist on the physical resemblance between Georgie and her cousins — although no one else really sees it. Georgie gets engaged to Mount’s friend David Dimbleby. When the engagement is abruptly called off, it’s Unca, not Georgie, who breaks the devastating news to Dimbleby. A pattern is established: as soon as one of Georgie’s relationships gets too serious, it is vetoed by her parents. Mount guesses at the reason. Munca can’t afford for anyone to get too close to the family in case they start asking awkward questions.

Long after Munca’s death, and long after Georgie learns, aged nearly 50, that she was in fact adopted, Mount sets out to discover who his aunt really was. This quest makes up the bulk of the book. Over the course of a decade, Mount orders up file after file from the General Register Office and the National Archives at Kew. He trawls the internet for clues and patiently pieces this fiendish-level jigsaw together. Which might sound boring — the textual equivalent of someone else’s holiday snaps — but the result is the most gripping book I’ve seen all year. I laughed. I gasped. I read it so greedily that I had to force myself to slow down towards the end.

Why is it so good? To start with there’s the story, which is genuinely incredible — I don’t want to give too much away. Let’s just say that fairly early on Mount learns that the man he thought of as Munca’s brother Buster was actually her son. A typical sentence from the chapter on Buster: ‘By the time he is 27, he has been married three times. He has also been an electrical engineer, a professional dancer and driven half a dozen different racing cars.’ There are so many twists that the narrative could easily be too convoluted to follow, but Mount traces his ball of string back through the labyrinth of Munca’s lives without a single tangle.

Along the way we get a generous, eclectic history of the early 20th century. Nothing is off topic as long as it’s interesting: Mount has an unerring eye for the compelling character study, the diverting anecdote. There are so many surprises, and each time you turn a corner you find yourself in a different bygone world. Motor racing and cricket in the 1930s; the steel industry and the back streets of Victorian Sheffield; the RAF in the first world war. There are cameos from Wallis Simpson and the Duke of Windsor, and Tom and Vivienne Eliot.

Mount never loses sight of the complex tragedy at the heart of Munca’s story. ‘The collateral human damage she inflicted over the years is hard to calculate.’ He correctly situates this damage in the harsher moral landscape of the past: ‘We have a duty to pity first before we comprehend.’ His empathy for the most minor of characters, his evident admiration for Munca’s chutzpah, his sensitivity to the myriad possible interpretations of any single clue from the archives — all of this is what makes Kiss Myself Goodbye unique and immensely enjoyable. I only wish it were longer.