It's All About the Duke (The Rakes of St. James #3)
Amelia Grey

Orphaned young and shuffled from family to family, Marlena counts on her fierce independence and quick wits to keep herself content. Being the responsibility of a notoriously wicked duke who upended so many lives is an unexpected challenge when she realizes he arouses her decidedly feminine desires. Marlena must be careful. She has her own scandalous secret to protect. If he finds out, will it shatter her chances of a happily-ever-after with the notorious rake?