John Legend and The Roots are such simpatico musical forces, it’s hard to believe that before this year, they’d never collaborated. Thankfully, the slickest soulman around and the hardest-working band in hip-hop have mapped out a smart (though safe) course for their collaborative project: an album of soul and R&B covers delving deep into the proud history of African-American protest anthems. Even while stretching out on an epic 11-minute version of Bill Withers’ incendiary anti-war polemic “I Can’t Write Left-Handed,” The Roots’ tight playing serves the songs and their messages rather than the other way around, while Legend has mastered the art of singing expressively without over-emoting. Legend is sometimes reserved and too smooth, but here, he finds a tricky combination of tasteful restraint and genuine passion, whether immersing himself in the down-and-out inner-city blues of “Hard Times” and “Little Ghetto Boy” or striving ecstatically, though quixotically, toward liberation on “I Wish I Knew How It Would Feel To Be Free.” It’s just a shame that, with the exception of a single original (“Shine”) that understandably doesn’t measure up to the high standards set by the covers, Legend and The Roots had to reach so far into the past to find songs that comment so powerfully and insightfully on the issues of today.