Barely a lady Drake's rakes series, book 1.

Olivia Grace has secrets that could destroy her. One of the greatest of these is the Earl of Gracechurch, who married and divorced her five years earlier. Abandoned and disgraced, Grace has survived those years at the edge of respectability. Then she stumbles over Jack on the battlefield of Waterloo, and he becomes an even more dangerous secret. For not only is he unconscious, he is clad in an enemy uniform. But worse, when Jack finally wakes in...