Music: The Blue-Collar Blues

It was an ordinary Chicago mailbox, the kind mailmen use for stashing their extra loads while making rounds. But what were those shuffling and humming sounds coming from within? Curious or startled passers-by probably never found out, but they were made by Mailman John Prine, scrunched up inside the empty box to escape the icy wind, eating his lunch and composing his mournful songs.

That was two years ago. Since then, Prine, now 26, has quit the post office, launched into a career as a songwriter and singer, and emerged from his box, so to speak, as one of the nation's most...