Very satisfying end to an excellent trilogy and a great book in its own right. Pen and Mark fall in love, and the mystery is wrapped up. My review has spoilers for the first two books, but I avoided spoilers for this one.
My cis opinion on the trans representation here: I read Pen as genderqueer and genderfluid. He never feels like a man or a woman, and his feelings about his gender presentation also change between days. He experiences gender dysphoria when people see him as male or female, and he dresses as he wants to be seen. So, he's happy as a trapeze artist, but he won't be able to live as himself if he becomes earl. Because earls are all men, people will see him as a man, and because the nobility are always under public scrutiny, he can't continue to rebel against gender norms. He is also attracted to men, and knows he'll have to ignore that part of himself as well.
One of my favorite things about this book is that Pen is quite selfish. He likes being adored, and although he does come to love Mark for who he is, he's drawn to Mark at first because Mark is drawn to him. Mark calls him brave more than once, sometimes in relation to his gender, and he is brave. But he isn't meant to be a lesson in bravery for cis people. He has to protect himself, and he isn't saintly. We know from the first two books that Pen needs to accept the earldom in order for Clem to keep his livelihood, and Justin and Nathaniel face danger because Pen has stayed hidden. So, the plot of Pen being reluctant to become earl doesn't work unless he's selfish. He does have probably the best imaginable reason to reject the earldom, but any reader reaching this point in the trilogy knows exactly what's at stake if he does that; Mark certainly does. All of this creates a very believable conflict between Mark and Pen.
Mark is also a great character. I was expecting more background about his mother, but she's only mentioned a few times. Having an anarchist for a parent turned Mark into a very practical man, because in his opinion, someone in the family needed to have their feet on the ground. He is bi/pan, and his orientation is pretty close to my own - he's attracted to certain traits in people and never concerned about gender. So I can say that this book works for me as great bi/pan representation. He doesn't struggle with accepting Pen's gender, although he has only encountered binary trans people before meeting Pen, so he does need to learn some things.
Mark's disability is also an important part of who he is, and he's modified his life to accommodate having one arm. The book sets up a parallel between Pen's gender and Mark's disability without making it seem like Pen's gender is itself a negative trait, because Mark mostly sees his own disability as a way that he's different from most people, rather than as a difficulty.
Overall I love this pairing - they have a major conflict, which we saw happen at the end of book two, but when they reunite, it's very romantic. At that point in the story, Pen really needs someone he can trust, and Mark is so solid and reliable. They laugh a lot in bed almost from the beginning, which is very funny and sweet. Clem and Rowley are still my favorite pairing from this trilogy, but Pen and Mark's bond is so natural and loving. I totally believe in their happy ending.
So, the mystery! I had no idea who the killer was until very far into the book. I think that's amazing for a three-part series. I suspected literally everyone, even the heroes from the other books! I also could not figure out how Pen and Mark were going to find happiness, but they do. So it's an exciting book, definitely gripping. There's also a nice side pairing, which I won't spoil, and all of the major characters from the previous books make appearances. Clem and Justin both play important roles, in ways that are completely fitting for them.
I don't know what's next for KJ Charles, but I'm looking forward to finding out. She amazes me every time. I also think she's elevated the whole genre by consistently writing inclusive books, and doing it so well.