Commonwealth, Ann Patchett's wonderful new novel, opens with a random occurrence that will ultimately shatter two families. It's a Sunday in 1960s Los Angeles. Fix Keating, a cop, and his wife, Beverly, are hosting a christening party. A latecomer shows up: Bert Cousins from the D.A.'s office, a man Fix barely knows and distinctly hasn't invited. The charming interloper carries a bottle of gin as a makeshift gift, and Fix finds himself pumping Bert's hand, welcoming him.