Bernard Cornwell - Sharpe's Sword. Richard Sharpe and the Salamanca Campaign.

Sharpe's Sword is a historical novel in the Richard Sharpe series by Bernard Cornwell, set during the Salamanca campaign of June and July 1812.
The novel sees Captain Richard Sharpe tasked with a crucial intelligence mission: to safeguard a vital British spy code-named "El Mirador," whose network of agents is essential for Wellington's campaign against the French. The greatest threat to the spy is the French officer Colonel Philippe Leroux, who is on a mission to uncover and silence him.
The story plunges Sharpe and his loyal friend, Sergeant Patrick Harper, into a murky world of espionage and intrigue within the aristocratic society of Salamanca. Sharpe seeks personal revenge against Leroux, who previously killed a British officer.
The narrative culminates in the Battle of Salamanca on July 22, 1812. After the battle, Sharpe pursues Leroux, forces a final sword duel, kills him, and finally claims the French officer's sword as his prize. The true identity of "El Mirador" is revealed to be an Irish professor and priest, and Hélène, Sharpe's lover, is revealed to be a French spy.