Written late in his life, J. S. Bach's The Art of Fugue has long been admired--in some quarters revered--as one of his masterworks. Its last movement, Contrapunctus 14, went unfinished, and the enigma of its incompleteness still preoccupies scholars and musical conductors alike. In 1881, Gustav Nottebohm discovered that the three subjects of the movement could be supplemented by a fourth. In 1993, Zoltan Goncz revealed that Bach had planned the passage that would join the four subjects in an entirely unique way. This section has not survived, but, as Goncz notes, it must have been ready in...