"No one ever seems to go in or out of that building," says Sean Wilentz, pointing out Princeton's Nassau Hall, a campus landmark old enough to have been held by the British during the Revolutionary War.

It's appropriate that this eminent American historian ("The Rise of American Democracy," "The Age of Reagan") is talking about spirits from the past and mysteries of the present. His new book, " Bob Dylan in America," (Doubleday) is about how the strains of American music and American history have come together in one man over the course of a nearly 50-year career. In Wilentz's view, Dylan has served as a conduit for potent and nearly forgotten strands in American musical, folk and political culture. The Popular Front artists, the Beat writers, the forgotten blues singers discovered by John and Alan Lomax, these are some of the people whose work speaks through Dylan. And so, appropriate for a historian, the book is a vision of how the past becomes part of our living present....