The Duke's Defiant Bride (Brides of Mayfair #4)
Michelle McMaster

Juliet Reed never wanted to love the handsome British army captain. After an unspeakable tragedy changes her life forever, the half-English and half-French young woman became a deadly spy for the French, known as Lady Blade. When she is taken captive by Captain Adams, the two enemies must fight against the heated attraction that burns between them, or betray everything they have fought for.
Now, a year after the Battle of Waterloo, the two former lovers––and former enemies––must re-unite in order to stop a dangerous plot which could cost many innocent lives. But with their volatile past and their dangerous passion for each other re-ignited, will the two star-crossed lovers risk everything to trust each other once more?