Confession time. Neither I nor my children were born recently enough to have been remotely interested in Harry Potter, and so when JKR entered the CriFi world with her Cormoran Strike books, I was only mildly interested. My loss. My mistake. ‘Bestseller’ is a fluid and relative term, much abused by hyperactive publicists, but if ‘Robert Galbraith’ has sold thousands of books, then good for her. She writes beautifully. Former military policeman turned private investigator Strike is “a broken-nosed Beethoven… over a stone off his ideal weight..” And with just half a leg.

The London PI has a deliciously fraught relationship with his business partner Robin Ellacott. He tries to push back thoughts that he is in love with her while, at the same time, mourning the suicide of his girlfriend, who slit her wrists in the bath because she knew what he knew. Meanwhile, Robin is unattainable, because of her commitment to her Met Police detective partner.

The latest case begins in a  bizarre fashion. Strike is summoned to a near-derelict house in rural Kent, where a rich restaurateur, Decima Mullins, conceals a three-week-old baby beneath her poncho. The baby’s father, she tells him, was Rupert Fleetwood, an impoverished but well-connected ne’er do well. She believes he is dead, murdered and mutilated in a botched raid on a celebrated London silversmith. Her problem is that the police have identified the body as that of someone else altogether, and Decima wants closure.

A subtle touch of genius is the way in which the early action is framed around the English conventions of Christmas. Galbraith avoids the obvious, but hints at the family tensions; singletons quietly dreading the few days back with mum and dad, despite the echoes of happier times; the monstrous extravagance of Harrods; the relentless joviality in pubs and bars, smothering – for a while – any sense of loneliness, loss and insecurity, yet all the while, making for an emotional hangover that is sure to descend once the TV adverts ditch snowflakes and baubles for the equally false promises of holidays under a Mediterranean sun.

This book is very, very long, at just short of 900 pages, but its length gives the author the space to write with great perception and detail, in an almost Dickensian or Hardyesque way, about the way the main characters react and speak to each other. In the hands of a lesser writer, this might make for laborious reading, but here, every paragraph is precious.

Galbraith introduces a touch of the esoteric into the plot, and what can be more esoteric than the arcane symbolism of Freemasonry? The original robbery which gave us the dismembered corpse of – well, there’s something of a queue to join this particular ID parade – involved Masonic silver artifacts, not intrinsically priceless, but of great significance to the initiates.

I must mention the quotes at the beginning of each chapter. Some are from an obscure manual for Freemasons, written by Albert Pike, but others, from Matthew Arnold, Robert Browning, AE Housman are more evocative. We also hear the voice of John Oxenham, a long forgotten writer from the early twentieth century.

Nagging away, like a persistent bass counterpoint under the main tune, is the situation between Robin, her Met boyfriend Ryan, and Strike. Ryan Murphy seems to be everything that Strike is not; conventionally handsome, deeply in love, resolutely honest and utterly devoted to Robin. Murphy wants the elusive first home, the hungry cry of an infant in the night. But what does Strike want, or offer? Robin tells us:

“He was infuriating, stubborn and secretive when she wished he’d be open. But he was also funny and brave, and he’d been honest tonight when she’d expected him to lie. He was, in short, her imperfect best friend.”

The plot is Byzantine in its complexity, and Strike and Ellacott scour the country from Sark to Scotland before they finally discover the identity of the ‘hallmarked man’ and who killed him.. The denouement – in an unremarkable terraced house in the West Midlands – is breathtakingly violent, but my abiding thought about this magnificent novel is, “My God, how did she do this?” The Hallmarked Man is published by Sphere and is available now.