WOBEGON BOY John Tollefson, the son of Byron and Mary of Lake Wobegon, leaves Minnesota for upstate New York, to manage a public radio station at a college for academically challenged children of financially gifted parents. Free from the Dark Lutherans of his hometown, he makes a pleasant bachelor life for himself in New York. He buys a new house and paints it a deep gold. He has a bright idea for a restaurant specializing in fresh produce. He falls in love with a historian named Alida Freeman. He is presented with public radio's coveted Wally Award.  In the midst of plenty, it occurs to John that his life lacks nobility and grace. A consumer of fine food and wine and giver of good parties, he yet has no coherent life story. Compared to his great-grandfather John Tollefson, who finagled his way over from Norway, he feels rootless, restless, joined in no struggle, with nothing at stake. The only true magnificence in his life is Alida, who eludes his courtship and gives him an impassioned speech about the pleasures of living alone.  Folded into the romance of John and Alida is the checkered saga of his ancestors - dour butcher, a playboy publisher, a medicine-show politician, Siamese-twin ballplayers, a Texas Pentacostalist, and a bank embezzler - and Lake Wobegon itself, with its bachelor farmers, its stout-hearted burghers and housewives, its simple code: Cheer up, Make yourself useful, Mind your manners, and Avoid self-pity. A useful code, as John discovers in his pursuit of magnificance, especially as the going gets tougher.