Unrestrained by Joey Hill is one of the first mainstream D/s romances I read. Til then, I had found the kinds of D/s I want to read in erotica, or books like Carried Away from small kink presses, but not in the romance section. Not that I could check out of the library. Not that I could get on Kindle. (Though some of these older titles have been digitized now!) This book opened the world of D/s romance for me, and I'm so glad. 
 Unrestrained has a lovely tipping point into D/s, and really luxuriates in the slow fall into deeper dynamic, lets the reader savor the joys of service, and that is why it has a special place in my heart. It's told from the POV of a cis straight woman, Athena, who was Mistress to her husband as an act of service and is now uncovering and exploring her submission in a more active way. (It's a bit sub for you in that this is sparked by the love interest in the story, Dale, but not so intensely that I was turned off by it.) 
 Kink details: 
 It starts with a Sir/girl D/s dynamic and develops toward M/s and 24/7, at a slower pace than others I've read, and in a less goal-oriented way than I usually see. Hangs out at Master/submissive for a good long while; that's where it is for much of the book. There is explicit negotiation and characters clearly think a lot about safety and also keeping the dynamic private outside of kink space. There's a lot of sexual play, including quite a bit of sexual objectification. It also includes bondage, pain play, and service. The kink takes place in community; the characters meet at a kink club and are active in kink community. There is a strong disciplinary thread in the D/s, quite a bit of punishment, and a bit of a resistance dynamic. The kink feels fairly realistic. (Hill is prone to magical mind-reading insta "natural" doms, and that is fairly curtailed in this story, much more than most of her other work. It does slip there on occasion.) 
 Notes about representation: 
 Dale is disabled and that is an ongoing thing in the story, in terms of navigating service and caretaking and his sense of masculinity and dominance with regard to it. It is not a book I would recommend on the strength of its disability representation. It tends towards unnecessary info dumps about and focus on his disability, it ties his disability very strongly to his insecurities around dominance, and Athena often acts in ableist ways that she can't see and dismisses his concerns about. That said, it also holds more complexity and nuance than most disabled dom characterization I've read, and her ableism does get checked enough that she starts to get the complexities of the situation a bit more. In general, characters and conflicts are deeply drawn in pretty complex ways, which I think really helps with representation like this. 
 Other things of note: 
 A deeply heteronormative m/f romance with very possessive monogomy. Athena is very wealthy and Dale is not; class is discussed but not a source of conflict, for the most part. Her social power in terms of wealth is a real thing in the book, the main relationships you see her have outside of the D/s are with her housekeeper and her secretary. Both characters are middle aged, something that's rather rare in BDSM romance. Dale is a retired Navy SEAL and that's a big part of his characterization and the story. There is a sense that he is part of a chosen family with other SEALs. The romance is front and center, with no other core plots other than a storyline around working through grief (Athena lost her husband to cancer). 
 Additional content warnings: 
 (view spoiler)[ Athena experiences some harassment over shifting from being a Mistress to a submissive. A couple detailed descriptions of combat. There are references to and one instance of sexual harassment. An attempted mugging near the beginning of the book, complete with rescue by her love interest. A long moment in the story where they are doing charitable work with poor kids and there are references to abuse and neglect. Dale runs a dog shelter and there are references to animals being harmed. (The dogs seem secure and content in the present, and we get to see that quite a bit.) (hide spoiler)]