This is a sequel to Daughter of Mystery, which I haven’t read, but works by itself. It would probably still be wise to read the other book first to get a fuller sense of two characters, Margerit and Barbara, who also play roles in this story.
The setting is a cleverly constructed one in which early 19th century Europe includes at least one additional country (Alpennia) somewhere wedge into that hilly corner near France, Switzerland, Austria and Italy/Piedmont. Magic is a real power but in the form of mysteries’ religious/catholic-like sacramental rituals with genuine if diffuse power, and also alchemy.
The focus of The Mystic Marriage, as is obvious from the title, is alchemy but set amid the personal and courtly affairs of a set of aristocratic women.
It did take me awhile to submerge into this world (possibly because of skipping to book 2) but once in and having got the characters clear in my head the novel had a a nice sense of intrigue and some moments of genuine tension. Maybe a bit too mannered for my tastes but skillfully executed.