City of Mirth and Malice by Alexis L. Menard

Some debts are not so easily repaid—or forgiven.

Nicolai Attano finally has everything he’s ever wanted: control over the Iron Saint Railway, safety for his family, and not a single rival syndicate to stand in his way. Never could he have imagined he’d want anything more than power—not until Camilla Marchese.
Six weeks have passed since the disaster on the train and his wife was kidnapped, but to get her back, he’ll need help. Nico struggles to work with a city poisoned against him and is instead forced to seek the aid of a disreputable black-market dealer. A simple bargain to ensure her rescue, Nico doesn’t think twice. A bargain is struck, and the cost is higher than ever. The collateral? His life.
Meanwhile, Camilla fights to gain control over her newfound powers as the alchemist in charge of her confinement puts her to the test. Forced to free what she’s held so close, the only light in this hellish prison is the hope to see her family again, and the husband who stole her heart. Three trials and three chances to find the source of her Chaos and use it to save herself, if it doesn’t destroy her first.

Cracks in the city draw lines between loyalties, and time is running out until everything comes crashing down. Return to Lynchaven in this action-packed and swoony conclusion of Nico and Milla’s story.