The Smoking Mirror — Helen McCloy

Celia McNeill is under arrest during her vacation to France and, desperate for cash, she makes a deal with card master Sergei Radetzkoy, whom she meets while being detained in Dieppe.

He says he's just lucky at cards, but the crooks who follow him from the casino aren't interested in his winnings. They demand to know his 'system'. The fashionable folk who rescue him don't conceal their interest in his extraordinary success at the casino either - an interest that leads to murder ...