The Barbed Crown

From the publishers:
 
In The Barbed Crown, the sixth tale of rogue and adventurer Ethan Gage by William Dietrich, our hero returns to Paris and London. Against a background of imperial pomp and the gathering clouds of war, Gage plots revenge on Napoleon Bonaparte for the kidnap of his son. Paris, the “City of Lights,” shines – but alongside its splendor is great squalor. Heroic patriotism rubs against mean ambition, while grand strategy and back-alley conspiracy are never far apart. While Ethan spies on the French court, his wife, Astiza, works to sabotage Napoleon's coronation using the Crown of Thorns, a legendary relic said to have come from the Crucifixion itself. But when Napoleon is crowned nonetheless, they flee to England.
At Walmer Castle on the English coast, Gage joins a daring campaign by Smith, Fulton, rocket inventor William Congreve and smuggler Tom Johnstone to halt Napoleon's intended invasion of England – a campaign which leads Ethan to take a role in the Battle of Trafalgar itself…