One of the most reticent and recondite musicians in St. Louis is also one of the most knowledgeable, skilled and in-demand. Though he'd blanch at the description, guitarist and singer Jim Nelson is to St. Louis folk music what Bob Costas is to baseball: He knows everything worth knowing -- from bluegrass to country to his deepest passion, old-time music -- and he expresses that understanding with wit, precision and irresistible pleasure. He has picked with John Hartford and the nationally recognized Volo Bogtrodders, produced and backed fiddler extraordinaire Chirps Smith and kept flawless time at a thousand contra dances. If you think folk music is Ani DiFranco, you won't get Jim Nelson. If you believe folk music is about genuine roots in timeless American music, you need to hear Nelson flat-pick his old Gibson guitar.