Castle on the Rhine — Caroline Farr

Prisoner of Evil. For the beautiful American, Susan Fletcher, a vacation cruise of the Rhine suddenly became a nightmare of terror when she was kidnapped undercover of darkness and whisked away to Castle Stahlemberg. Imprisoned in this isolated, gloomy fortress, tormented by the insane master of Stahlemberg and his diabolical henchman who insistently demanded information she couldn't supply, Susan tried every trick she could think of to win her freedom. For after all, who could she, a stranger in this country, turn to for help?

Only Claus Biemann, the handsome fellow passenger with whom she had been dining that fateful night, even knew she was missing. And Susan didn't know if Claus felt as strongly about her as she did about him; if he would or could discover what had happened to her. So she waited in her small dark cell, trying to plot her escape, praying fervently that Claus -- or anyone -- would find her before it was too late...