Herbie Hancock is a musical chameleon. Although he is primarily known as a virtuoso jazz keyboardist, Hancock has never been afraid to venture outside of the jazz world in a career that has often stretched far beyond the boundaries of the genre. This sense of both adventurousness — and what would appear at least to be more than a little bit of musical restlessness as well — has led Hancock in a number of different directions over the years.

In the seventies, Hancock briefly enjoyed broader mainstream success by releasing more funk-fusion based albums like Headhunters and Thrust, both of which were commercial hits. He repeated this again in the eighties by collaborating with turntablist Grandmixer DST on the early hip hop crossover smash "Rockit," from the album Future Shock. Just last year, Hancock once again did the genre-busting act by releasing the album Possibilities, an album of collaborative efforts with pop stars ranging from John Mayer and Annie Lennox to Christina Aguilera and Sting.

Like the Possibilities album, Hancock's new River: The Joni Letters is also an album of collaborations with famous pop stars like Norah Jones and Tina Turner and some not-so-famous. However, unlike the former album, the sound here couldn't be further away from pop, as Hancock turns his musical ear back towards jazz to interpet the songs of another musical chameleon, the incomparable Joni Mitchell.

From the first song on this disc, a torchy version of Mitchell's "Court And Spark" with a smoldering vocal by Norah Jones, it becomes clear that what Hancock seeks to capture on this loving tribute is the essence of Mitchell's artistic ingenue. For her own part, Jones is more than up to the task of channeling the jazz-based atmospherics of Mitchell.

Hancock clearly sees Mitchell as more the chanteuse of albums like her jazz-influenced masterpiece The Hissing Of Summer Lawns than the more folk-influenced artist of her earlier years making pop hits like "Big Yellow Taxi." In fact, on one of Mitchell's best-known songs here, "Both Sides Now," Hancock eliminates the vocal altogether. In doing so, Hancock also turns the flowery optimism of the song more inside on itself. Here it is transformed into a more introspective-sounding tone poem, where the instrumentalists are allowed ample room to stretch.