This was first published in 1986 as The Shadow in the Plate and is set six years after the previous volume The Ruby in the Smoke, this novel takes place in 1878. I know that they tended to go in for long engagements in the past, but six years seems like an awfully long time for nothing to have changed between Frederick and Sally. Indeed, it's like things have actually gone downhill. They are frequently at odds and outright name-calling arguing in this volume, so perhaps the long-term outcome was all for the best.
The dark stories continue with both Frederick, who is inexplicably a private investigator now, working with Jim, and Sally tackling different ends of what turns out to be the same problem. Sally, now with her own financial advisory business and a large dog, is trying to help a client recover the three thousand pounds which she lost after investing it on Sally's advice. The company went bust and Sally just knows that it wasn't any accident or poor planning. On the contrary: the collapse of the company was planned in detail by Axel Bellmann.
Meanwhile, via Jim, a showman and magician Alistair Mackinnon has had death threats. Mackinnon supposedly has the power of psychometry - being able to divine things from touching objects, and through this he has become aware of a murder. At a séance conducted by Nellie Budd, Jim and Fred learn of the very death which Mackinnon has seen. Evidently Nellie has psychic powers despite the fraudulent medium game she pursues.
Bellman sends a lackey to threaten Sally, who works alone out of her home. He has documented many visitations from men - obviously seeking financial advice, but Bellman plans to spin it as a house of prostitution if Sally doesn't back off. Sally doesn't back off.
To further his interests and influence, Bellmann plans on marrying the daughter of Lord Wytham. I have two observations here. The first is purely regarding my own amusement when I read this sentence: "Lord Wytham was a handsome man" to which I wanted to append, "Lord without 'em he was ugly as sin," but that's simply frivolity. It does, however, offer an insight: you should be careful how you write things, and also how you choose your character names if you don't want to provoke unintended mirth amongst your readership! Moreover, why were his looks important? No answers are to be found here.
The second thing relates to this with regard to the complementary sex (not opposite, surely!) in describing female characters as beautiful. It's almost like there's a law forbidding female characters from being ordinary or plain. It seems that male characters - even major ones, in novels can get away with any amount of ordinary and average, yet females are required to be young and beautiful - not pretty, not attractive, not good-looking, although these do occur, but outright beautiful. I think it's a poor choice and worse, a clichéd choice against which I've railed on more than one occasion
I want to give here, thanks to Philip Pullman, an example of how it can be done and made to work well. Frederick, the photographer, has his breath taken away by Lord Wytham's daughter, Lady Mary. The text reads, "...beautiful wasn't quite the word. The girl was astoundingly lovely, with a grace and shyness and delicate coral coloring which made him want to reach for his camera..."
So here is the first part of it - a photographer's view. Note that it's not the author telling us she's beautiful, but a character observing her to be so, and he's doing this because he is a photographer - someone who we would expect to react to beauty whether it's in the face of a woman, or in a sunset, or a flower, or something else.
Later, another character says to the main character, Sally Lockhart, "...Lady Mary's beauty would fade. Yours is not dazzling, but it is a beauty of mind and character, and it will grow stronger...." To me, that is exactly how it should be approached and how it can be done well. Anything else is cheap by comparison and insulting to women in general.
In addition to Sally, there is another strong woman in this novel - she's an ardent admirer of Mackinnon's who has no illusions about her own lack of beauty. Her face is disfigured by a birthmark, but she shows her inner beauty by how strong she is in the face of her poverty and in her lack of a more ordinary-looking face. She is the one who shows them a newspaper clipping which confirms the visions both Mackinnon and Budd have had. It's someone Bellmann killed in a duel. We also have confirmed something which has been a growing suspicion for attentive readers: that Mackinnon is actually the son of Lord Wytham and Nellie Budd.
Sally has by now learned that Bellmann is building an automated steam gun. His belief is that once every nation owns these guns, peace will inevitably reign because no one will dare start a war. He's delusional of course, as the arms race between the US (United States) and the US (Union of Soviets) conclusively proved. The big guys simply pay the little guys, one way or another, to fight proxy wars. As long as there are haves and have-nots, war is inevitable. But this is not the problem with the steam gun as Sally discovers. It's confined to railway tracks. With such limited mobility, Sally determines that it's intended to be used against a nation's own population, not against foreign aggressors. But Sally has a plan.
Pullman evidently likes to kill off main characters with the glee of a Joss Whedon or a Jo Rowling, and he manages to slaughter both Sally's dog and her fiancé, as Frederick is by then. Bellman is also dead, and we're left with the knowledge that Sally's one brief dalliance with Frederick has borne fruit. I recommend this as a worthy read.