The Duke in Question by Amalie Howard (Mass Market)

This lady is not at all a paragon of society…

Lady Bronwyn Chase leaves London on her brother's passenger liner bound for America with a packet of secret letters that could get her into trouble. Serious trouble—the kind a duke's sister shouldn't be in; the kind that puts spymaster Valentine Medford, the Duke of Thornbury, on her trail. But as the duke gets closer to Bronwyn and the secrets she's keeping, he'll have to decide between the mysterious woman who calls to him, or his allegiance to the Crown.