Nell Gwynne's Scarlet Spy — Kage Baker

Lady Beatrice was the proper British daughter of a proper British soldier, until tragedy struck and sent her home to walk the streets of early-Victorian London. But Lady Beatrice is no ordinary whore, and is soon recruited to join an underground establishment known as Nell Gwynne's. Nell Gwynne's is far more than simply the finest and most exclusive brothel in Whitehall; it is in fact the sister organization to the Gentlemen's Speculative Society, that 19th-century predecessor to a certain Company...and when a member of the Society goes missing on a peculiar assignment, it's up to Lady Beatrice and her sister harlots to investigate.