So much so is this particular time period popular that a lot of books run together in plot. Willig's books differ in that they fit into historical fiction more than historical romance. They have a slight flavor of a regency romance over a true romance novel.
In The Crimson Rose, Mary Alsworthy is approached by Lord Vaughn to work for the Pink Carnation. In return for being bait to lure the Black Tulip (the Carnation's archnemesis) out into the open, all Mary wants is for Vaughn to fund the next Season for her. Why? Pride. Mary's younger (and homelier) sister recently wed Mary's former suitor, and the brother-in-law has offered to fund the next Season. Mary, however, doesn't wish to be seen as an object of pity, so she'd like her Season to be funded in another way.
Lord Vaughn has good reason for picking Mary: her black hair is one hallmark of agents that work for the Tulip. Mary is also a good screen for Vaughn, who has been suspected by many, including those in the Pink Carnation's circle, of being the Black Tulip.
Of all the couples in the books, I like Mary and Vaughn the best. They are not perfect. Mary can be cold and manipulative, well aware of the fact that as a woman, her status depends on making a good marriage. She wants to be comfortable, and she realizes that comfort and love are not the same thing. Vaughn is also sarcastic and has a dry wit; the two often trade barbs. Vaughn also has the requisite black past, complete with dead wife, dead mistresses, and membership in various and sundry hellish clubs. His reputation is hardly pristine, and he is well aware of it.
Many of my comments on this book are the same as the other books. The book is a light read and doesn't fit cleanly into one genre. It's a combination of chick lit and historical fiction, with a dash of mystery. It's very light on history, with not a lot of substance, mostly the outer trappings. It should not be read to get a deeper understanding of the time period.
I'm not sure where Willig is going with the series. We know who the Pink Carnation is. Her historian, Eloise, knows who the Pink Carnation is, and she's well on the way to a Happily Ever After herself. Willig introduced an element of the Jacobite Rebellion, when Bonnie Prince Charlie attempted to take his "rightful" place on the throne of England.
Since I liked the main characters, which is what really counts, I give the book an A.