Georgette Heyer 
These Old Shades [EPUB ebook] 

Justin “Satanas” Alastair, the reclusive Duke of Avon, is known for his ruthless reputation and dark past. One night, while in Paris, the Duke encounters a mistreated street urchin named Léon Bonnard. Taking pity on the boy, the Duke purchases him and makes him a page, unaware that Léon is actually a young woman named Léonie, the daughter of the Comte de Saint-Vire who had once wronged him. Alastair plans to use his new personal servant to get his revenge on an old enemy