Mine at Midnight by Adele Clee

When Miss Eliza Dutton discovers her first case for the Order involves helping a lord who has a gambling habit, her instinct is to refuse. The fact her father lost his fortune and his life at a gaming hell is the reason she despises Lord Roxburgh. But Eliza is given an ultimatum: help the handsome rogue or find work elsewhere.
Lord Roxburgh has a reason for trying his luck at the tables, and it has nothing to do with winning money. But his life of dissipation is brought to a swift end when a guest is found murdered in his garden.
While working together to solve the crime, will Eliza discover there’s more to Lord Roxburgh than his sinful good looks? Can a lord who’s suffered the worst kind of tragedy abandon old habits and fall in love?