Historical fiction is handled with a casual, epigrammatic sophistication in this truly affecting recreation of one of the great affaires de coeur--the Mayerling story. It is written in the form of Rudolf, the Archduke of Austria's Commonplace Book, a daybook beginning the year before his death. Rudolf's marriage to the querulous Stephanie is one of ""uneasy torpor;"" he is given no real authority by his father, whom he defies here and there. There is a good deal of history along with the romance from the time when Rudolf is first attracted by the enigmatic innocence of the young Maria, asks her to accept a love ""without definitions,"" then carries it beyond the strictures of family, church and dynasty as he attempts to have his marriage annulled by the Pope and the Rota. Whether or not this was ever more than a temporal temptation, it turned into an enduring love affair, and, in the hands of a new novelist, it's an imperially elegant entertainment for young romantics.