This one comes courtesy of my friend Rachael Herron.
In 1902 Miss Cora Strayer opens her own detective agency, advertising it with this irresistible line: "When in need of legal or confidential advice, why not confer with one of your own sex?" Irresistible hook, right? Even better, self-described nerd Paul Reda has done quite a bit of research already. A collaboration, perhaps?
Jackie Winspear has a similar thing going in her Maisie Dobbs series, but I think there's room in this world for a variety of feisty lady detectives, especially those who still make time for the gentlemen despite advancing age (cough-cough stella hardesty cough-cough).