Secret History of the Pink Carnation / Lauren Willig 388 p.

Eloise, an American graduate student, is on the chase for the identity of the Purple Gentian, a English spy a la Scarlet Pimpernel during Napoleon's reign so she can finish her dissertation for Harvard.  The story alternates between the spying and romancing of Amy Balcourt and the Purple Gentian and Eloise's Colin--a roadblock to reading his aunt's archive of materials about her family--royal spies in the court of Napoleon.

1st in the Pink Carnation series.