Oh, no! A cliffhanger! I don't remember a cliffhanger in any of her other books, and I've read them all. A mystery to be solved in the last book of each series, yes, but nothing like this.
Katherine Parish, orphaned at her mill-owner father's death, is invited to live with her mother's sister's family, the Cathcarts. She feels terribly out of place in this gentrified society. Kent Atherton is the Earl of Rennington's third son. He's at loose ends, especially since the family discovered that the murdered chaplain who married his parents was not ordained, making all of their children illegitimate. He yearns to find a calling, to make his own way in the world. He is especially fascinated with machines, and longs to see them, learn how they function, and work with them, if possible. His father resists this passion of Kent's, wanting him to stay in the home and telling him that gentlemen do not do that sort of work. Katherine was filled with the kind of knowledge Kent craved, and this, plus his good nature and cheerful manner, attracted them to each other. They are both easy to love, even when Katherine goes a bit over the top with her zealotry. In previous books, we had to learn to like Izzy and Bea Franklyn, who were not especially likable to begin with. (I still didn't warm to Izzy.) But with Katherine, we're cheering for the underdog.
Amusing secondary characters, like Mrs. Vance, from the mill town where Katherine had lived until her father's death, and Mrs. Ryker, to whom she was sent to be a companion, add interest, humor, and danger to the plot.
Captain Edgerton and his crew continue trying to unravel the mystery of the chaplain's death. Not only have they made little progress, but the whole investigation is made more complicated by the death of Miss Peach and the shocking event at the end of this book. The final book of the series will bring everything together, but we'll have to wait for Ambition to be released in September.
There are many characters in Mary Kingswood's books. She provides some helpful family trees on her website, in order to remember them all, as well as a summary at the beginning of the book of the pertinent characters. It isn't unusual to find names from previous series, and it's best to read the series in order.