Ethel M. Dell 
The Serpent in the Garden [EPUB ebook] 

When lovely Gabrielle Dermot left the shelter of the English vicarage where she had been brought up, to go to the Italian Riviera, three people awaited her with very different feelings. Her mother, who had left England years before with Count Gaspare Voltano, dreaded her daughter's arrival at the Count's villa, which to her had become a hated prison; the Count himself was eager, for he knew of Gabrielle's young beauty, and knew too that she could be a valuable assistant; Pierre Ronceau of the French Secret Service waited for Gabrielle's admission to the inaccessible villa above the Point des Sirenes to round out one of the most hazardous cases of his career.