Hazard a Guest by Ava Devlin

Ember Donnelly is not a woman who needs rescuing.
Ten years ago, she turned a derelict gambling hell into one of the most infamous establishments in London, and she’s outplayed every man who’s tried to take it from her—until now. When a ruthless rival makes a dangerous play for all she holds dear, Ember must seek out aid from the last person in the world she wants to ask for help.
Joseph Cresson is a quiet, meticulous barrister with a well-known blushing weakness for the formidable Miss Donnelly. But a year in revolutionary Lisbon has left him stronger, steadier, and surprisingly difficult to fluster. He’s still a man of principle—he’s just no longer afraid of breaking a few rules to do what’s right. Like keeping Ember safe. Or finally telling her how he feels.

The plan is simple: avoid detection by the enemy, stop a hostile takeover, and keep an errant earl and a mischievous debutante out of the trouble served up on every passing platter at a party like this one. Joe and Ember are about to learn that at Blackcove, even the sharpest players can’t rig every game—and when hearts are on the table, all bets are off.