Improvisation rattles some listeners. Maybe they're even suspicious of it. John Coltrane's saxophonic flights of fancy, Jimi Hendrix's feedback drenched guitar solos, Ravi Shankar's sitar extrapolations--all these sounds seem like so much noodling or jamming, indulgent self-expression. "Just" improvising, as is sometimes said. For these music fans, it seems natural that music is meant to be composed. In the first book of its kind, John Corbett's A...