Herbie Hancock, River, The Joni Letters (Verve). Without its cadre of vocalists, Hancock’s tribute to Joni Mitchell would not have received a Grammy nomination or widespread critical attention. In varying degrees, Mitchell, Norah Jones, Tina Turner, Luciana Souza and Corinne Bailey Rae do justice to Mitchell’s songs. Leonard Cohen is effective in his atmospheric delivery of her lyrics in “The Jungle Line.” But if the CD contained only instrumental tracks of the quality of “Both Sides Now,” “Sweet Bird,” “Solitude” and “Nefertiti,” it might be the best Hancock-Wayne Shorter album ever. As it is, River is one of their finest collaborations.