The affair of the mysterious letter

Upon returning to the city of Khelathra-Ven after five years fighting a war in another universe, Captain John Wyndham finds himself looking for somewhere to live, and expediency forces him to take lodgings at 221b Martyrs Walk. His new housemate is Miss Shaharazad Haas, a consulting sorceress of mercurial temperament and dark reputation. When Miss Haas is enlisted to solve a case of blackmail against one of her former lovers, Captain Wyndham finds himself drawn into a mystery.