Like a comet that periodically returns and lights up the sky, the Will the Circle Be Unbroken series manages to be predictable and illuminating at the same time. Once again, the Nitty Gritty Dirt Band draws an assembly of deities and wannabes around the campfire. Some of these are familiar, though veterans from the first volume have grown scarce: Doc Watson plays "I Am a Pilgrim" as a tribute to the late Merle Travis, who cut the same tune on the 1972 Circle session. Jimmy Martin returns too, his galloping, almost-reckless delivery undimmed by the years. From the 1989 sequel comes Johnny Cash, whose "Tears in the Holston River" eulogizes Maybelle Carter, his mother-in-law and the soul of the first Circle. New faces take the place of those who have departed: Willie Nelson is a logical addition, though his duet partner, Tom Petty, sounds uncomfortable and awkward on "Goodnight, Irene." Emmylou Harris assumes her place in this pantheon, her voice breaking hearts even in harmony with Matraca Berg on "Oh, Cumberland." No performance stands out more than that of Taj Mahal, whose presence has a demographic significance and whose rollicking rendition of "Fishin' Blues nearly steals the show. But Mahal also contributes to the album's only serious blemish: On the inevitable title cut, he and the other soloists play with a solemnity that deletes the song's communal energies. A congregational enthusiasm distinguished its performance on the first album; here, the singers -- particularly Alison Krauss -- pass it along, verse by verse, as if it were priceless china. This music is about soul, not trepidation, so it's to everyone's credit that such moments are scarce here. Let's hope that they don't dress it up with string samples or breakbeats once Will the Circle Be Unbroken, Vol. 4 rolls around.