The prospect, at any rate, is alluring--however mundane the mystery. The New Jersey Nelsons have come to Vezelay to spend the summer in the cottage Mom has just inherited, sight unseen, from old Uncle Armand--Mom and 16-year-old Jan with romantic visions, Dad with trepidations, 13-year-old Tony cavalier as usual. And to Mom's consternation and Dad's fury, the place turns out to be a shambles--uninhabited for 30 years since, the Nelsons subsequently learn, Armand's brother Jacques was mysteriously murdered there. Murdered, apparently, for something that has never been found. Plot-wise, it's a hand-overhand pull to the end: the accidental discovery of a rare, ingeniously concealed stamp. (They never do learn who killed Jacques.) But meanwhile the Nelsons have cleaned out and fixed up the cottage, made assorted cosmopolitan friends (one of them a love-interest for Jan), and grown quite accustomed to baguettes, jambon, and fromage. Et voila! with the $100,000 the stamp brings, they can keep the cottage--plus, plus, plus. OK as a pipe dream, then; otherwise a drag.