Alexis Hall’s 2019 The Affair of the Mysterious Letter is a standalone fantasy mystery.
Wounded in the war against the Empress of Nothing, Captain John Wyndham returns not to Ey, the straitlaced home of his youth, but to his college town, the cosmopolitan city of Kelathra-Ven. Despite years spent away at the front, John is at heart still a small-town boy.
He ends up rooming with the flamboyantly decadent sorcerer(ess) Shaharazad Haas. Haas appreciates Wyndham for his placid toleration of Haas’ eccentricities.