Newt's emerald - Garth Nix

Inspired by the works of Georgette Heyer and Jane Austen, Garth Nix's Newt's Emerald is a light hearted Regency romance with a fantasy twist.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 moustache-wearing man. During Truthful's dangerous journey she discovers a crook, an unsuspecting ally, and an evil sorceress but will she find the Emerald?