The Venetian Affair by Helen Macinnes
While on assignment in Paris and Venice, an American journalist works desperately to expose Communist espionage activities.
Fenner burned Rosenfeld’s message, reminding himself wryly that he was behaving in the very best tradition. This was a game not too difficult to learn, he thought. A game? A game in deadly earnest. A vacation in Venice that was grim business. A girl constantly beside him who wasn’t his. How the hell had he walked into this upside-down world? Where, he wondered suddenly, would Venice lead?