The Seduction of the Crimson Rose is the 4th book in a series by Lauren Willig. The books follow hapless grad student Eloise Kelly as she tries to uncover the identity of the Pink Carnation and other similarly-named flowery spies (think the Scarlet Pimpernel). The books all have two levels: Eloise and her research and her ever-deepening romance with a decendant of one of the flowery spies, and a past romance Eloise uncovers in her research. The books are light and funny romances with a bit of intrigue and danger.
This particular one follows Mary Alworthy, a beautiful and wily girl who lost her best chance at a good suitor to her sister, as she teams up with the slimy and seductive Sebastian, Lord Vaughn. The more time Mary and Vaughn spend together trying to ensnare the Black Tulip (a.k.a. the bad guy), the more they are drawn together. But when Vaughn's dead wife shows back up, and the Black Tulip draws ever nearer, things get a little complicated.
Fun series. Start with The Secret History of the Pink Carnation. They are better in order