Robert Galbraith (Sphere, £20)

Each chapter of this fourth novel in the Cormoran Strike detective series is prefaced with a quote from Henrik Ibsen’s Rosmersholm, a play in which the central character, Rosmer, expresses support for a new reformist government only to be accused of betraying his upper-class roots. Rosmersholm is also concerned with guilt, the breakdown of trust and being driven to suicide, and its political points are often expressed through its characters’ personal relationships, so it’s an apt comparison for J.K. Rowling (does anyone actually call her Robert Galbraith?) to make.

Lethal White begins where 2015’s Career of Evil left off. Private detective Cormoran Strike – former military policeman, illegitimate son of a rock star, missing half a leg after an explosion in Afghanistan – has become a minor celebrity for catching a serial killer. His assistant-turned-business partner, Robin Ellacott, now sporting a nasty scar after their last adventure, is finally tying the knot with her clearly unsuitable fiancé, Matthew. At the wedding, she discovers that Strike’s pleas to come back and work for him again have been deleted from her phone by her new husband, setting the tone for a marriage in which trust and respect are conspicuous by their absence.

Strike, meanwhile, is alternately fretting over his current romance and harking back to old flames. But this is all an elaborate masquerade: the engine driving this series along is the unresolved sexual tension between Strike and Robin, and the plot constantly threatens to take a back seat to the duo’s efforts to hide their mutual attraction behind case files and stakeouts. Nevertheless, it works. The tension between their jobs and their personal lives is one of the defining characteristics of the Strike novels, and Rowling is careful to see that the plot never suffers as a consequence.

And eventually a plot does emerge, sparked by the arrival of a disturbed man in Strike’s office claiming to have witnessed a murder. Before long, Strike is hired by Jasper Chiswell, an MP who is being threatened with blackmail, though for what he refuses to disclose. Robin goes undercover in the House of Commons to gather information, but her stint in the corridors of power is merely a prelude to the intrepid pair’s encounter with Chiswell’s family, a dysfunctional, squabbling clan wedded to the codes and traditions of the upper class, who have their own inscrutable motives for hiring a detective agency.

By around the fourth book in the Harry Potter series, Rowling’s novels were starting to show the need for a firmer editorial hand. The Potter books, each longer than the last, were becoming bloated with extraneous material, and Rowling’s prose style had lost some of its lustre and elegance. Weighing in at 647 pages, Lethal White is the longest Cormoran Strike novel so far, and yet doesn’t feel overstuffed, meandering or verbose – more like a confidently-written, well-paced character-driven detective story that takes its time to come to the boil. Whether Robert Galbraith novels would be topping the bestseller charts without the big J.K Rowling reveal, we will never know. But judging from Lethal White’s merits, they’ve earned their place.