These Old Shades by Georgette Heyer
This is a rather fun story. Justin Alastair, Duke of Avon, buys a young boy named Léon to be his page when he literally runs into him on the street. Except Léon is actually Léonie, which he suspects from the start.
She falls rather in love with Monseigneur, and Justin, much to everyone's surprise, including his own, falls in love with her. There's a somewhat convoluted revenge scheme, wherein Justin uses Léonie against his mortal enemy. He plays the Hamlet gambit, and it works.
Léonie is a wonderful, mischievous character. She spent many years pretending to be a boy (in her brother's inn, to protect her from lascivious guests), and she doesn't take well to being made into a girl, with girl clothes, etc. She does admit they make her look pretty, but she'd still rather wear breeches.
Justin and Léonie are Vidal's parents in Devil's Cub.
I enjoyed this book. Fans of Heyer or historical romance should enjoy it, too.