Let's just say if you require a lot of accuracy in your historical fiction, this is probably not necessarily where you want to be reading.
Sands' heroine was married without having a Season after her father nearly lost the estate to gambling and somebody showed up to marry off his eldest daughter and pay the debt. The daughter (secretly) had a substantial dower from another relative, and after the marriage, the man stops with the Oh You Are My Lovely Rose and switches to being a verbally but not physically abusive asshole who isolates her from her family and friends. But you don't really get to know him in person, because he is mostly present in the book as an inconvenient body, a la Weekend at Bernies or The Trouble with Harry.
Spoilers, ho! If you don't want spoilers, well, leave before you are shipped off to the Colonies as an indentured servant. Or something.
We never do find out who poisoned the asshole husband. Apparently that is divulged in book 2 or 3, which I probably won't read, because this is one of those retell it from the other person's perspective things and a lot of the later books repeat the earlier books.
I haven't read other books by the author, so I don't know if her relatively heavy-handed pop psychology is typical. But it is relatively heavy-handed. It's not that I disagree with her; it's that I have trouble imagining the historical characters thinking that way.
Aside from the unsatisfactory resolution re: who poisoned the asshole, the book is a romp. Lots of misunderstandings and physical comedy, especially surrounding intimate encounters. The twin of the dead husband and the heroine really do wind up having sex in part on a bed where they have forgotten they have stashed the dead guy. Which is why I read the book in the first place, so if that makes you snort, this is probably a good choice for you. Otherwise, probably give it a pass.