The heroine, Jane, likes sex and is adventurous about it with her husband, which for the setting is pretty impressive in itself. The hero's valet, Robert, is gay and submissive, and has found himself a situation where he can fulfill those aspects of his character through a perceived obligation to the hero; his conflict involves breaking free of that obligation to go to the man with whom he's fallen in love (a secondary character from previous books in the series). The hero, Lord Minshom, was the villain of previous books. (Yay for villains becoming heroes!) He has serious angst from childhood abuse, but though that is part and parcel of his personality and he thinks that's the main reason he's kinky, it doesn't seem to be really true. When, as is inevitable in a romance, he's confronted most of his issues and is happier, he's still kinky; it's clearly shown that he and Jane are going to enjoy themselves to the fullest with each other and with a male friend.
The main plot revolves around conflict between Minshom and Jane. They've been married for ten years but estranged for the last seven; in the interim, Minshom has only had sex with men. (In the previous novel, he was the poisonous-yet-charismatic top from whom the hero had to break free.) I guessed pretty quickly what the issue had been between them, but still enjoyed how it was revealed, bit by painful bit. Minshom wants Jane to go back home and never see him again; Jane wants several things which I won't spoil here. They banter and battle and manipulate, mostly in sexual ways but not entirely, and though Jane is younger, they are still pretty evenly matched and I suspect she could out-kink her husband if she tried.
Overall? I had a lot of fun reading this. Not just as an erotic romance, but as a commentary on the tropes of Regency-set historical romances.
Though set in the Regency, most of the events in this series take place in a House of Pleasure where they can experiment sexually, or in various sexual situations. There are a few ordinary social occasions portrayed (in this book, there's a ball, and a trip to a modiste), but they're not the focus, and the books aren't intended as interpretations of Regency society; I think they're more psychological explorations of relationships. That said, I love when small historical details are included; for instance, one character is a captain in the Navy, and the way he's described always makes me think of the naval officers in Persuasion.
I'm looking forward to the next one!