The Golden Spaniard By Dennis Wheatley

Spain writhed in the torment of Civil War. In a Madrid bank lay ten tons of gold: and both sides wanted it. Its owner was the lovely Countess Lucretia Coralles – who, known to the rebels as ‘the golden Spaniard’, led the double life of a secret agent.
She had other secrets too – and one of them was the reason why the Duke de Richleau responded so quickly to her appeal for help, throwing himself into the desperate struggle in which his own closest companions of earlier adventures were now his most dangerous foes. . . .