When he emerged from the nightclubs of Greenwich Village, Bob Dylan was often identified as a "protest" singer. As early as 1962, however, Dylan was already protesting the label: "I don't write no protest songs," he told his audience on the night he debuted "Blowin' in the Wind."

"Protest" music is largely perceived as an unsubtle art form, a topical brand of songwriting that preaches to the converted. But popular music of all types has long given listeners food for thought. Fifty years befor...