Bob Dylan, according to Sean Wilentz’s passionate and informative (if at times lurchingly uneven) new book, “has dug inside America as deeply as any artist ever has.” This is well put, for it suggests the way in which Dylan’s songs (there are now more than five hundred of them) seem to unearth a strange, alternate, subterranean America, an antic shadow country of dirt roads and frontier towns, abandoned mines and teeming plantations, a land inhabited by outlaws, vagabonds, crapshooters, confidence men, vigilantes, and religious fanatics, to name only its most conspicuous citizens.