The Pale Assassin — Patricia Elliott

Eugenie de Boncoeur is growing up in Paris, unaware that her guardian has contracted her to marry the sinister spymaster known as 'le Fantome' when she turns sixteen. She finds herself falling for the handsome lawyer, Guy Deschamps, but there is little time for romance; France is descending into chaos as the Revolution takes hold. Soon Eugenie is fleeing for her life. Her brother Armand has become involved in a plot to save the King from the guillotine, the mob is searching for aristocrats, and le Fantome, the pale assassin, is on their trail - desperate for revenge.