The Lion's Skin - Rafael Sabatini

Justin Caryll was an unwilling hero: an unwilling plotter of revenge, an unwilling spy and an unwilling pursuer of the woman he loves. Justin must thread a perilous path through various sticky wickets, between the machinations of his foster father, the violence of his brother, the relentless pursuit by the government's agent, his hopeless love for the hapless heroine, and the struggles of his own conscience. Fortunately, Justin is equipped with the skill and speed to lunge and parry, both verbally and with a sword, to make his way through 18th century London with plenty of aplomb, in the midst of Jacobite plots and financial finagling by the ultra rich.