On April 30, Willie Nelson turned 70, celebrating with the release of his latest greatest-hits collection. The Essential Willie Nelson (Columbia/Legacy), a two-CD set, has an intriguing 1970s-vintage cover shot that sets exactly the right tone for forty years of selective tracks. Nelson's unkempt long red hair and scraggly beard frame his thin, almost Bob Hope nose. His mouth twists slightly, a smile just short of a sneer, in sardonic, knowing reaction to the world behind the camera. His eyes, couched in wrinkles and bags, stare straight and deep into the lens, and suggest hard-to-fathom distances and recessions at the same time as they focus you into connecting. This interaction, evasive, seemingly casual, direct and subtle, represents the essence of Nelson's sly, almost unobtrusive art.

The Essential Willie Nelson demonstrates once again that the Red-Headed Stranger's nonchalant, gospel-flavored, jazz-inflected voice and guitar have remained essentially themselves for decades despite a wild variety of musical backdrops: bare-bones string bands, sleekly glossy Nashville productions, twangy 1970s Outlaw country-rock, jazzy standards with strings, gospel-laced soul.

Maybe that breadth helps explain why Nelson's recurrent duets with Ray Charles are almost always so charged--and so much fun. After all, only Charles and Bob Dylan have traveled as sure-footedly across as far-flung a constellation of genres and expectations as Nelson has and still remained themselves; Charles and Nelson have long shared material and appearances. (A 1984 show at the Austin Opera House, captured on The Willie Nelson Special [Rhino Home Video], features excellent versions of "Georgia on My Mind," "I Can't Stop Loving You" and the old hillbilly fave "Mountain Dew.") Part of this odd couple's magnetism stems from the fact that they represent opposite poles of the American spectrum. Charles, the consummate besuited black professional trained in the tough world of low-rent, postwar r&b, whose nonpareil voice influenced countless singers, is a hard-bitten recluse who heads a thriving business dynamo and a drilled band. Nelson, the white country-music renegade who tried pig farming for a while when his career soured, comes onstage in hippie-cowboy-Indian costume, calls his band Family and is the epicenter of the self-consciously laid-back Austin music scene (his disciples include Townes Van Zandt, Lyle Lovett, Joe Ely, Jimmie Dale Gilmore and alt-country outfits like Uncle Tupelo) he helped seed and feed for the thirty years since he first strode onstage at Armadillo World Headquarters. Even on an off night, Charles can suddenly burst whatever frames the ragged, churchy elasticity of his scuffed and soaring trademark voice. Nelson stays introspective: Lacking Charles's explosive interpolations, he subsumes his surroundings, enticing the audience into his voice's unexpected contours.