Duchess by Night, Paperback/Eloisa James

A Mischievous Charade... Harriet, Duchess of Berrow, is tired of her title and the responsibilities that come along with it. Enough with proper tea parties and elegant balls; what Harriet really wants is to attend an outrageous soiree where she can unleash her wildest whims and desires. But to attend such an event--especially if the event in question is Lord Justinian Strange's rollicking fete, filled with noble rogues and rotters, risque ladies and illicit lovers--would be certain scandal. That's why she must disguise herself... Looking forward to a night of uninhibited pleasure, Lord Strange is shocked to discover that beneath the clothes of a no-good rake is the most beautiful woman in the room. Why is a woman like her risking her reputation at his notorious affair? And can he possibly entice her to stay... forever?