Well, I believe I have finally found a Romance novel I liked. Of course, the fact that it's half Mystery might have something to do with it.
I think that author balanced both subplots very well. She also did her research. It's not just the facts; the language fits the setting as well. Never have I read a Historical Romance before that painted the actual atmosphere rather than just dressed characters in period costumes and placed them on the appropriate stage. At least, that's what I felt. (I should note, however, that I'm far from a great Historical Romance or Romance reader in general.)
I have encountered few female characters I liked as much as Mrs. St. Cloud. She is strong without being an action girl or very sharp-tongued. Her independence, unusual for the time period, has well-thought-out reasons. And she is smart. Not only book-smart - she can think on her feet as well.
There is a number of sex scenes in this book, but they fit well with the rest of the plot. Of course, we can't have one of those without purple prose but here it wasn't nearly as bad as it can get.