The Broken Heart by Mary Lancaster

Danger, loyalty and loving the enemy…
Shunned by society and even by her own family after the treachery of her late husband, Isabelle de Renarde has long given up any thought of love or remarriage. But desperate loneliness drives her to make an assignation at the Hart Inn – which is where, inevitably, everything changes.
For the Hart is in turmoil once more, captured by a small group of French raiders led by the wildly reckless Captain Armand le Noir – who always succeeds in the end. Dazzled by Isabelle’s beauty, Armand regards her at first much like the task in hand – a distraction from his own pain. But the Hart seems to work its magic and inconvenient attraction gets in the way of both unwise assignation and the search for lost prisoners of war.
With the raiders only partially foiled, the adventure sweeps from the inn to a neighborhood ball and then to France itself.
Amidst the constant danger and the intensity of their growing love, Isabelle and Armand live on the edge of betraying each other or their own loyalties.
But someone is manipulating them, the true enemy is unclear, and in the end they must both risk everything to be together.