Paganini showman, womanizer, dazzling virtuoso is one of the most charismatic characters in the history of classical music. His violin, il Cannone (the Cannon), is now kept in Genoa, Italy, where it is played only once every two years in a sold-out concert by the winner of an international competition. This year, though, a Parisian art dealer is found dead in his hotel room the day after the concert. In his wallet is a scrap of sheet music, torn from...