Dark Angel by Tracy Grant
Adam Durward is an outsider in both his mother's India and his father's England. Too much of an outsider for his childhood sweetheart, Caroline, who turned her back on their forbidden love to marry the wealthier, safer Jared Rawley. Taught a bitter lesson about not belonging, Adam left England as a diplomat and spy in the Peninsular War. But even then he could not escape Caroline. When he learned her husband, Jared, had betrayed crown and country, he exposed Jared as a traitor despite Caroline's pleas.
Sheltered, pampered Caroline grew up and found unexpected courage in the wake of her husband's disgrace. She left her decorous life in England and followed Jared into war-torn Spain. Now a widow with a young daughter, she is trapped behind enemy lines. Adam sets out to rescue the woman who still haunts his dreams. In a landscape set with treachery and intrigue, Adam and Caroline's only hope of survival is to rely on each other. They brave bandits, enemy soldiers, and harsh terrain, but the greatest danger they face may be navigating the web of love and betrayal that still binds them together.