The Masque of the Black Tulip — Lauren Willig

Modern-day graduate student Eloise achieved the academic coup of the century when she unmasked one of history's greatest spies, the Pink Carnation, who saved England from Napoleon. But now she has a million questions about the 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. When she finally comes across an old codebook, Eloise discovers something juicier than she ever imagined: an unlikely pair who were hot on the trail of the Black Tulip and had every intention of stopping him from killing the Pink Carnation and bringing down England. But what they didn't know was that while they were trying to find the Tulip--and stumbling upon something like love--the Black Tulip was watching them....