I'm not sure what my review will add, as my criticism seem to be pretty typical for readers of this book, the third in the Penhallow Dynasty series.
My expectations for this book were high, as the situation/tropes presented in the summary were right up my alley: childhood friends in a marriage of convenience, and I enjoyed the earlier books in the series. I also found Hugo to be an enjoyable and sympathetic character in the previous book. However, right off the bat, it's not exactly the presented situation: Hugo and Katherine knew each other for one summer as kids, when she was around 7-8, and he was around 12-13 (I couldn't decide if my timeline were off, or if the author loses track herself) and weren't particularly close. It's also never really stated why they were buds, when Hugo has several younger siblings that Katherine might have meshed with better. There's also the loose thread of how Katherine's parents are determined for her to be nothing less than a duchess, but somehow, a Penhallow (of any impoverished, untitled sort) is an even better coup than a duke. For some reason?
The characterization of the hero and heroine, while not totally unsympathetic (to me, ymmv as you see from the other reviews), is unsatisfying and doesn't really hang together. Katherine is presented as this extremely damaged, prickly individual, but really she just has gauche, embarrassing, rather indifferent parents. Hugo never develops at all from his previous characterization in the last book: easy-going, gregarious and family-focused. He basically decides that he has to marry an heiress to save his family, does (incredibly easily, lost opportunity for romance development there), and then seemingly decides that he will love and desire Katherine, since he's already married to her, anyway, and hasn't been getting regular sex while overseas as a soldier. Most of his softer emotions towards her are presented as a sort of condescending pity versus any romantic love or attraction. Hugo's sister is jealous of Katherine, but otherwise, his family just accepts her and doesn't seem to think much of her--a little resentment or even just plain awkwardness would be more realistic.
Katherine's motivations are presented as getting away from her parents/assuming control of her own life and having this vengeful Season, to compensate for feeling foolish during her previous one. Both these goals are accomplished with ease and varying levels of satisfaction on her part. In theory, you could ascribe her arc from defensive, lonely shrew at the beginning to a brief run as society darling to the sweet and modest wife, mother, and author as character growth, but it comes across at times as completely different characters. Hugo, on the other hand, is flat, simple, and shallow, if perfectly . . . fine.
When Katherine's situation (and thus to a certain extent Hugo and his family's) changes, everyone just kind of shrugs and moves on. Even though it was considered dire that he marry her? I know he has his own money at this point, but everyone involved is just very casual about this marriage as a whole.
I dunno. A disappointment based on the previous books, and my attraction to the premise. I also didn't enjoy Hugo's sister, so between the two, I haven't bothered to move on to the next book in the series.