C.S. Harris’ long-running series of historical mysteries feauturing aristocratic amateur sleuth Sebastian St. Cyr reaches its nineteenth instalment with What Cannot Be Said, in which our hero is asked to investigate the murder of a noblewoman and her daughter. With the mysteries around Sebastian’s own heritage and family circumstances now resolved, the last few novels in the series (as well as this one) have more of a standalone feel about them – although I do think it’s an advantage to have some knowledge of what has gone before.

It’s a sunny afternoon in July, 1815, and two brothers are in Richmond Park enjoying an afternoon away from the hubbub at home caused by their sister’s upcoming wedding. When the air is rent by two pistol shots, they immediately wonder if there’s a duel happening and race to find out, but they’re completely unprepared for what they find – a woman and a girl lying in the grass next to the remains of a picnic, their arms crossed over their bloodied chests, their bodies feet-to-feet and the air filled with the stench of fresh blood and burnt gunpowder.

The two victims are Lady Laura McInnis and her sixteen-year-old daughter, Emma, but more disturbing even than the murders themselves is the way the bodies have been posed in exactly the same way as the victims of another double murder fourteen years earlier, that of Julia and Madeline Lovejoy – the wife and seventeen-year-old daughter of Sir Henry Lovejoy, Bow Street Magistrate. But their killer, a traumatised ex-soldier, was apprehended and hanged for the crime – so could these latest murders be the work of a cruel copycat? Or, as Lovejoy is beginning to fear, could they have executed the wrong man?

I always enjoy the complexity and historicity of these mysteries; there is a lot going on beneath the surface and the author never shies away from showing us the more unpleasant and distasteful aspects of what life was like for ordinary people in the London of the early nineteenth century. Although cushioned by his station as a member of the aristocracy, Sebastian is not unaware of his privilege and is regarded by many of his peers as a dangerous radical as a result, and his wife Hero (now expecting their second child) is well-known as an advocate for social reform through her investigations and publications detailing the conditions endured by the less fortunate.

Through Hero, Sebastian learns of Lady McInnis’ work on behalf of the poor children of the city, of how she stood against the awful practice of baby farming (the upper echelons sending their by-blows to be ‘fostered’ but in effect to be killed off) and of the barbaric (and frequently life-ending) use of very young children as ‘climbing boys’ to clear chimneys of soot and debris. It seems Lady Mcinnis earned herself quite a few enemies from all strata of society – from chimney sweep to the Regent’s favourite bastard and even her own husband – by speaking out as she did, any one of whom could have had a motive to kill her. But as the pool of suspects grows, there is one thing that offers room for doubt. These people may have had reason to want Laura McInnis dead – but Emma McInnis? Why would anyone want to kill a girl not yet out of the schoolroom?

Once again, Sebastian finds himself wading through the murky underbelly of English society while at the same time going up against those of his own class who try to fob him off with lies, half-truths and threats in their attempts to prevent him from discovering what really happened. Among the latter is his powerful father-in-law, Charles Jarvis, cousin to the Prince Regent and his closest advisor, with whom Sebastian has had many a run-in and who is not above using violence to get his own way.

While Sebastian himself is an attractive and complex hero, one of the strengths of this series is the regular supporting characters, many – if not all – of whom have been present in the majority of the other books. Hero is always firmly in her husband’s corner; they’re a great match on every level and that fact absolutely shines in their scenes together. Lovejoy and Sebastian may have got off to a rocky start (in the series opener, Sebastian was wanted for murder and Lovejoy was tasked with apprehending him) but now share a strong friendship and sense of duty. I enjoy seeing Sebastian’s improving relationship with his father, the Earl of Hendon, and his aunt Henrietta, one of the doyennes of the ton and purveyor of useful gossip, is always good value. Surgeon Paul Gibson, one of Sebastian’s oldest friends, is frequently called upon to perform post mortems – although in the last few books, we’ve witnessed his increasing dependence on opium for pain relief (he lost the lower half of one leg to a French cannonball) and Sebastian’s growing fears for his friend’s life.

The author skilfully incorporates the different characters and relationships into the story without taking focus away from the mystery, and once again weaves lots of fascinating historical detail seamlessly into the story. What Cannot Be Said definitely feels like one of the darker entries in the series, given the spotlight the author shines on the inhumanity exhibited by so many towards the defenceless, and the glimpses we get of the utterly terrifying ‘treatments’ for mental illness espoused at the time - and the final twist in the dénouement is shocking and tragic, both in terms of the solution to the whodunnit and what happens in the aftermath.

C.S. Harris has once again crafted a compelling, complex mystery and combined it with lots of fascinating historical detail to produce another riveting instalment in what is, surely, the best historical mystery series around. I’m sure fans of the Sebastian St. Cyr series will need no encouragement to dive in – and, as I said at the beginning of this review, the book works as a standalone, so new readers will find much to enjoy here, too.