Before getting into what I didn't like, I want to start with what worked well for me. Sadly, there isn't much.
What I LIked
1. The Pleinsworth Sisters: Sara, Harriet, Elizabeth, and Francis Pleinsworth belong to an off-shoot of the Smithe-Smith family and are the main character's (Iris) cousins. Sara was the heroine of the third book in the series and she remains my favorite Smithe-Smith heroine. In reality, these girls would probably drive me insane within a matter of hours, but on paper they're so much fun. Harriet considers herself to be a playwright and in this book she premieres the play she was writing in the last book (something about Henry VIII and a Unicorn). Francis is an 11 year old spitfire, who just loves unicorns and likes to think that she is one, something that was probably aided by Iris gluing a horn to her head for her performance in the play.
2. The Talk: This is a scene that's been done hundreds, perhaps thousands, of time in historical romance--the heroine's mother comes to her on the day of her wedding and petrifies her about The Thing That Will Happen. This is no different here, but what I really liked was that almost immediately after Iris's mother gives her The Talk, Sara comes and gives her another, more realistic one, even telling her that she and Hugh had sex before they were married. This was a great scene and I loved the connection between Sara and Iris and it made me wish that there were more scenes involving these two in the other books.
What I Didn't Like
1. Daisy Smithe-Smith: I didn't like her in any of the prior books, but I tended to think of her as just another deluded Smithe-Smith girl, who thought she was the next great musician. In this book, I learned that she's the epitome of the snobby teenager that I hated in high school and really really hate now. You're probably thinking that she couldn't be that snobby, but she is. For instance, there is a scene in which she, Iris, and their mother were talking about Pride and Prejudice and she criticizes Elizabeth Bennett for turning down Mr. Darcy when she was lucky he even looked at her. Then, a few pages later while talking with Winston Bevelstoke about his sister (heroine of another Quinn book) and criticizes her for refusing to marry a prince and marrying someone who is not a prince.
2. Iris's Lack of Self-Esteam: Iris considers herself to be on the shelf, which she probably was for that time period (how insulting is that image?) and she doesn't think that men pay any attention to her. This is something that Quinn has done really well before (Penelope in Romancing Mr. Bridgerton owns her spinster status and uses the fact that no one pays attention to her to further herself), but it falls flat here. Compared to Penelope, Iris has no agency. She does what other people expect her to do and doesn't even try to fight it when she's forced to marry Sir Richard. (I know this is a fairly accuracy depiction of what would have happened in that time period, but considering the only people who saw her being kissed by Richard were her aunt and cousins, she could have gotten away with not marrying him. Her family, especially the Pleinsworth girls, would never have said anything to "ruin" her. Even Daisy would keep that secret because in ruining Iris she would be hurting her chances of making the advantageous match that she so desperately desires.)
Bonus: What Made This a Wall-banger:
Sir Richard: Our "hero" is a ginormous ass-hat. I suppose we're supposed to feel sorry for him because he's broke and raising his teenaged sisters, but all of that is negated by how he treats Iris, who he allegedly likes. He sets out, from the very first page, to trap a wife and is only attending the Smithe-Smith Musicale because he knows that all of the musicians are supposed to be unmarried ladies and he figures that one of them would be happy to marry him. One by one, he disqualifies the girls--Harriet and Daisy are too young and Sara is already married (she's the only piano player and agrees to play in the musicale despite hating it and skipping out the year before)--and he settles on Iris, who attracted his attention by being the only one who could actually play her instrument.
In ways, Richard is a very old-school hero. He sees Iris not as a person to love but as a means to an end. Instead of a woman, she is a big Pound Sign in a dress, despite not having a very large dowry. That's all he cared about--not that Iris is funny and smart. Page after page, I held out for hope that he wouldn't go through with his plan, but unfortunately, I was destined to be disappointed.
It also turns out that I was destined never to find out Sir Richard's titular secrets because I couldn't finish reading this.