The Grand Sophy, by Georgette Heyer.
Hmmm. I may not restrict myself to a single Heyer, but this would have to be in my top three. I've just been listening (again) to an unabridged audiobook of it, which is terrific and highlights the wit and irony of Heyer's writing. And the utter fun of the story.
This image came from here; it's the edition I have of the book (but am too lazy right now to photograph and upload...). That blog entry compares Sophy to Jane Austen's Emma. Sophy being Much cleverer at managing people.