Fosca was the last novel of Iginio Ugo Tarchetti, an Italian writer working in Milan in the middle of the 19th century. Other works include Paolina, Una nobile follia and Storia di una gamba. Written in 1869, Fosca was only translated into English in 1994, after the premiere of Sondheim's Passion. I am very interested in reading this book, but haven't had a chance to do so yet. Until I add my thoughts, you can find an interesting analysis of the novel in comparison to the musical at Astrid's Passion Page.