I'm midway through Paul Williams' massive, three-volume Bob Dylan: Performing Artist series, and I'm enjoying it immensely.
Williams makes the convincing argument that the primary unit of the Dylan ouevre is the "performance," and that those performances need to be considered in the same light as a jazz performance, with the same focus on improvisation and the inspiration (or lack thereof) that comes moment by moment.
I'm buying it. Anyone who has ever seen Dylan in concert knows that he is likely to transform even his greatest and best-known songs into almost unrecognizable cover versions. And anyone familiar with the wealth of material on The Bootleg Series sets has heard alternate takes on the officially released songs that essentially constitute new songs. The words may (or may not) be the same, but the arrangements and the overall "feel" of the music is completely different.
He also makes the convincing case that Bob Dylan is the greatest, most nuanced singer of the rock 'n roll era. But you all already knew that, right?