This book reminds me a little of Jacqueline Carey's Kushiel series. Like Carey, Murphy creates an alternate Europe with different names. Murphy's lands are fixed firmly in the Elizabethan era. She also focuses heavily on court intrigue and has a highly sexual heroine. However, I think Carey does all of that much better. Still, the book is enjoyable.
Belinda is the bastard daughter of the queen of Aulun. She is trained from birth to be a spy and an assassin. Eventually, she is sent to find or create a pretext for war with another country and possibly murder the queen.
It's her first time trying to blend in as a noble rather than someone from the lower classes. She finds friends, and the line between her true identity and her cover identity begin to blur.
She also discovers that she has forbidden magic power, a trait she shares with the prince, the son of the queen she was sent to kill. The discovery binds them together. The power is also shared by Belinda's father and other bastard heirs, and she begins to see plots within plots.
Using the witchpower also makes Belinda feel a need for sexual dominance. This results in some pretty explicit sex scenes, including one rape and at least one scene with dubious consent. Societal rules about sex are a little different, too. There doesn't appear to be any social stigma for two women having sex, but possibly at least one of the partners needs to be of a lower class and noble women are certainly still expected to marry and produce heirs. There's no indication either way whether sex between men is socially acceptable.
There are at least a couple of hints that the series is going to contain some scifi elements.