When twenty-five-year-old Bob Dylan wrecked his motorcycle near Woodstock in 1966 and dropped out of the public eye, he was already recognized as a genius, a youth idol with an acid wit and a barbwire throat; and Greenwich Village, where he first made his mark, was unquestionably the center of youth culture. In Positively 4th Street , David Hajdu recounts the emergence of folk music from cult practice to popular and enduring art form as the story...