Newt's Emerald by Garth Nix

Inspired by the works of Georgette Heyer and Jane Austen, Garth Nix's Newt's Emerald is a Regency romance with a fantasy twist. New York Times bestselling author Gail Carriger calls it "charming; quite, quite charming." 

After Lady Truthful's magical Newington Emerald is stolen from her she devises a simple plan: go to London to recover the missing jewel. She quickly learns, however, that a woman cannot wander the city streets alone without damaging her reputation, and she disguises herself as a mustache-wearing man. During Truthful's dangerous journey she discovers a crook, an unsuspecting ally, and an evil sorceress—but will she find the Emerald?