The Masque of the Black Tulip (Pink Carnation #2) by Lauren Willig

Harvard grad student Eloise Kelly achieved the academic coup of the century when she unmasked the spy who saved England from Napoleon. But now she has a million questions about the Pink Carnation's deadly French nemesis, the Black Tulip. And she's pretty sure that her handsome on-again, off-again crush, Colin Selwick, has the answers somewhere in his archives. But what she discovers in an old codebook is something juicier than she ever imagined.