With the twenty-first century only a few years away, it is sobering to realize that what most of us call "modern music" is so very old: Stravinsky's Rite of Spring, still shocking to many, is nearly eighty, while Debussy's Afternoon of a Faun, with which, according to the arch-modernist Pierre Boulez, "modern music awakened," is now closer to Papa Haydn's time than to our own. Yet controversies still rage, with composers quarrelling over aesthetic...