One of Texas’ most celebrated country songwriters, Robert Earl Keen grew up on hillbilly and bluegrass music. Those influences frequently surfaced in his gritty country offerings, yet Happy Prisoner comes full circle, as Keen revisits the music of his youth with an album of old-timey and bluegrass-infused covers. Keen doesn’t make the return journey alone. Guests include guitarist Peter Rowan, Lyle Lovett (who adds vocals to Jimmie Rodgers’ 1920s classic “T For Texas”) and Dixie Chick Natalie Maines (who appears on a languid “Wayfaring Stranger”). Plus, Nickel Creek fiddler Sara Watkins and banjo master Danny Barnes add sweet touches throughout. The disc stomps to life with the hillbilly offering “Hot Corn, Cold Corn,” which segues perfectly into what’s certainly Keen’s boldest cover: a deft tackling of Richard Thompson’s “52 Vincent Black Lightning.” With re-envisioned old-time classics, rousing bluegrass soirees, prison songs and lamentable murder ballads, Keen offers up a welcome and enjoyable change of pace.