Two books have arrived about a couple of songwriters who changed the way we think about what a song can be.

Until the 1960s, songs were generally written by one group of people who then handed their work over to another group who did the singing. Unless you're a scholar, the chances are you don't know what Gershwin or Porter sounded like. But if you have a pulse, you've heard their music performed. We often refer to a large portion of this body of work as The Great American Songbook. During its peak, people talked about these songs coming from Tin Pan Alley in New York City.

The exception to this rule, of course, was our folk tradition, where people made up songs about their lives and then passed those songs along orally to friends and fellow travelers.

To make a long story short, this division broke down in the early 1960s, when folk music crossed over to the pop charts during a "folk revival" that set the stage for the rock music phenomenon that was about to follow.

At the vanguard of the folk explosion was Bob Dylan. Dylan wrote his own songs and many of those songs became hits. At first, Dylan wrote pieces that were riffs on traditional folk sources, reworkings of blues and bits by the likes of his hero, Woody Guthrie.

But it wasn't long before Dylan discovered that a song, like a poem, could be about anything he wanted it to be. At this moment, the nature of American songwriting underwent a radical shift.

Revolution In the Air: The Songs of Bob Dylan, 1957-1973 by Clinton Heylin chronicles Dylan's creative output, one song at a time, and in detail over the period that saw Hibbing Minnesota's Bobby Zimmerman become "the voice of his generation."

Heylin, whose biography of Dylan has been widely praised for its scholarship, is a rigorous guide to the gist of what has made Dylan such a formidable figure. The songs are what Dylan - whose personal life remains a cipher - is really about. And these songs, by any measure, comprise an amazing body of work that, with some reason, has often been described as Shakespearean in its scope.

Whether you choose to read Heylin's book straight through -- from Dylan's teenage beginnings to what becomes the brink of the creative rebirth marked by his Blood on the Tracks album - or by dipping in here and there and following your nose from the story behind one song to the next, you're bound to learn a lot about Dylan's life, influences, his creative practice, and the various scenes he found himself in, not to mention the music business of the time.

Revolution In the Air, like its precursor, Ian MacDonald's Revolution In the Head about the Beatles' recordings, is an invaluable guide to have by your side as you traverse this monstrous body of work.

Dylan would have a profound impact on a fellow folksinger following in his immediate wake, Joni Mitchell. But after finding the permission she needed in Dylan's work to make songwriting her own, Mitchell would light out for a frontier of her own making.

Michelle Mercer's Will You Take Me As I Am: Joni Mitchell's Blue Period blends conversational prose with wide-ranging scholarly insight to make a compelling case about why we should pay attention to an important and audaciously original body of work.

Mercer is fascinated by Mitchell's approach to and, ultimately, dismantling of, the autobiographical as engine for her art through a sequence of albums beginning with the extraordinarily naked collection of songs on Blue in 1971 and concluding with Hejira in 1976.

As with Revolution In the Air, the emphasis here is on the work. But, in Mitchell's case, the work is practically inseparable from an extraordinarily complex life, rife with entanglements involving the likes of Leonard Cohen, James Taylor, Graham Nash, Sam Shepard, to name but a few.

Mitchell is an artist who reserved for herself rights - creative, erotic, questing - that are still more commonly associated with men. This made her an anomalous voice at the time; it still does.

Mercer's book is enhanced by the access the usually reticent Mitchell gave her. The interview material here, while slender, is, nevertheless, illuminating - a good thing given Mitchell's eloquence and deeply original sense of her own intentions.

Like Dylan, Joni Mitchell is a sacred monster. How lucky for us to happen to be sharing their orbit.

Will You Take Me As I Am: Joni Mitchell's Blue Period
Michelle Mercer
$24.99 Free Press

Revolution In the Air: The Songs of Bob Dylan, 1957-1973
Clinton Heylin
$29.95 Chicago Review Press