Have you discovered Georgette Heyer yet?
Yes? Which one is your favorite?
No? Well. I hope you'll look her up, and come to love her as I do.
I don't particularly have a favorite - there are so many wonderful choices, and they are all delightfully unique.
But I'll recommend These Old Shades as a good possible place to start.
Here's the 2002 Library Journal review on the audio version of this book, originally published in 1926 (original cover to the right):
Justin Alastair, Duke of Avon, is called "Satanas" by enemy and friend
alike. In the aristocratic circles of both London and Paris he has a
reputation as a dangerous and debauched rake. He has the occasional odd
whim, however, and when a semistarved, ragged child literally falls
into his arms on a dark Paris street, Justin purchases the miserable
scrap from his abusive guardian and makes the child his page. Cleaned
up and properly dressed, Leon proves to be surprisingly comely, with
delicate features and flaming red hair. His delicate manners coexist
with a personality that combines innocence and arrogance with a fiery
temper and a willful stubbornness. Strangest of all, he bears a strong
resemblance to the Comte de Saint-Vire, an old enemy of Justin's.
Before long Leon is unmasked as the girl Leonie, but the mystery of her
heritage deepens. The strength of the characters rather than the plot
fuels this novel. Justin cuts a dark and brooding hero figure; his
motives are definitely dubious at the beginning of the story and remain
questionable. Even the secondary characters are strongly drawn,
distinct, and important to plot development.
The book holds up. Remarkably, wonderfully, perfectly.
Heyer is known as the Queen of Regency, but often her books are set in other eras, and this one is actually Georgian, taking place in the court of Versailles.
In the likely event that you decide you're hooked on Georgette Heyer after a couple of sublime hours in her company, you may want to have Devil's Cub on hand, the also-wonderful sequel to These Old Shades.