In an interview this year, pianist Herbie Hancock described River: The Joni Letters  then still just a work in progress  as "a portrait of Joni Mitchell's music and life." To a certain extent, that description simply explains why, in addition to such Mitchell classics as Both Sides, Now and River, the album also includes the Wayne Shorter instrumental Nefertiti and Duke Ellington's Solitude, which Mitchell heard, as a child, sung by Billie Holiday.

"That's one of the early influences on Joni, Billie Holliday, and that influence is really in her voice," Hancock explained.

So what we have here, then, is less a tribute album than a sort of explication in which the pop icon we thought we knew is revealed as a jazz artist in disguise. But even though River flows in with the expected raft of guest stars, Norah Jones, Tina Turner, Leonard Cohen and even Mitchell herself among them, it's hardly an attempt at crossover. Indeed, it's even less a pop album than Mitchell's jazz-oriented Don Juan's Reckless Daughter and Mingus.

Interestingly, Hancock's album ignores both of those, instead drawing most heavily from The Hissing of Summer Lawns and Blue. But that's largely because what Hancock and company do here isn't imitate Mitchell's sound so much as illuminate her songs, revealing jazz content most listeners would have missed. It isn't just a matter of expanding on the pastel harmonies and elongated, leading phrases of Amelia, which sounded fairly jazzy in its original version; Hancock so completely transforms Both Sides, Now that you would think it, like Nefertiti, had been plucked from Miles Davis's late-sixties songbook.

A lot of that doubtless has to do with the company Hancock keeps. In addition to his own piano, the album's main instrumental voice is Shorter (who, like Hancock, was in Davis's storied late-sixties quintet), with a rhythm section built around another Davis alumnus, bassist Dave Holland, and two members of Hancock's current touring band, guitarist Lionel Loueke and drummer Vinnie Colaiuta. All five have experience both with electric instruments and pop rhythms, but for the most part they keep things acoustic and understated, emphasizing the quiet conversational cast of Mitchell's music.

That's particularly obvious in The Jungle Line, where instead of Burundi drums and a singsong melody we get Leonard Cohen intoning lyric-like poetry, so we hear the music within Mitchell's words  just as Hancock's unaccompanied piano reminds us of the melody within the original's thrumming percussion. But it works the other way, too, as when Mitchell, in Tea Leaf Prophecy (Lay Down Your Arms), renders each phrase as if it were dialogue from a real-life musical. Few singers in jazz or pop can phrase or use spaces between notes as effectively as she does.

Of course, we would expect Mitchell to shine on an album of Joni Mitchell songs, but she's hardly the only one to sparkle. Tina Turner works wonders with Edith and the Kingpin, bringing enough grit to the verses to make their small-town feel almost palpable, and Corinne Bailey Rae brings such depth and resonance to River that it's hard to believe the song is older than she is.

In the end, though, it's the songs, not the singers, that matter most, and on that level, Hancock's greatest achievement is to turn the familiar into something completely fresh and new. With River, it really is possible to hear both sides of Mitchell, now.