The Benevolent Society of Ill-Mannered Ladies / Alison Goodman.

A high society amateur detective at the heart of Regency London uses her wits and invisibility as an 'old maid' to protect other women in a new and fiercely feminist historical mystery series from New York Times bestselling author Alison Goodman. Lady Augusta Colebrook, "Gus," is determinedly unmarried, bored by society life, and tired of being dismissed at the age of forty-two. She and her twin sister, Julia, who is grieving her dead betrothed, need a distraction. One soon presents itself: to rescue their friend's goddaughter, Caroline, from her violent husband. The sisters set out to Caroline's country estate with a plan, but their carriage is accosted by a highwayman. In the scuffle, Gus accidentally shoots the ruffian, only to discover he is Lord Evan Belford, an acquaintance from their past who was charged with murder and exiled to Australia twenty years ago. With Lord Evan injured and unconscious, the sisters have no choice but to bring him on their mission to save Caroline. What follows is a high adventure full of danger, clever improvisation, heart-racing near misses, and a little help from a revived and rather charming Lord Evan. Back in London, Gus can't stop thinking about her unlikely (not to mention handsome) comrade-in-arms. She is convinced Lord Evan was falsely accused of murder, and she is going to prove it. She persuades Julia to join her in a quest to help Lord Evan, and others in need-society be damned! And so begins the beguiling secret life and adventures of the Colebrook twins.