Despite an amazing five-decade career, Herbie Hancock remains an enigma to most. But only the great Miles Davis can lay claim to reinventing jazz with crossover pop appeal as many times over. From hard-bop wizardry with Davis' second great quintet in the '60s (some of the best music ever played) and subversive '70s funk (man, that shit is still fly) to advanced technological experimentation with the '80s synthesizer-centric MTV hit "Rockit" (great for its nostalgia) and modern electronica in 2001's Future 2 Future (a serious miss), pianist/composer Hancock continues to steer clear of a comfortable course. His latest effort, River: The Joni Letters (Verve), acts as Hancock's debut foray into vocal music (with reinterpretations of Joni Mitchell tunes), and includes a dream team of guest spots, including Norah Jones, Tina Turner, Corinne Bailey Rae, Leonard Cohen, and Mitchell herself. A quintet lineup featuring veteran players Wayne Shorter and Dave Holland combats the potential cheese associated with pop rearrangements, resulting in a successful and listenable marriage of straight-ahead jazz with conventional sounds.