The Vanishing Thief by Kate Parker
Georgia Fenchurch appears to be an unassuming antiquarian bookseller in Victorian London, but the life she leads is as exciting as any adventure novel. For Georgia is a member of the Archivist Society, a secret association of private investigators led by the mysterious Sir Broderick.
When a frantic woman comes to Georgia claiming her neighbor, Nicholas Drake, has been abducted by the notorious Duke of Blackford, Georgia and the Archivist Society agree to take the case. But Drake is no innocent – he is a thief who has been blackmailing many of the leading members of London society. Georgia must infiltrate the upper crust – with the help of the dashing but dubious Duke of Blackford. She wonders which of them is using the other while the stakes increase to put lives in danger.