Georgette Heyer, who died in 1974, was absolute monarch of her small but fertile field of the Regency romance, that lovely fluff produced for those who harbor an 18th century fantasy of eloping to Gretna Green. This, her last novel, is a more serious work, set in the Middle Ages at the height of Lancastrian power, beginning with the infancy of the children of Henry of Bolinggroke and left unfinished by the author before the death of Bolingbroke as Henry IV. Miss Heyer chose to view the maze of political, military and court events through the career of John, Henry IV's third son, but in general the slant is influenced by Shakespeare. Henry IV is haunted throughout his reign by the death of Richard II, whose crown he usurped, and the death, though never solved here, is forecast by the King's words: ""I would he were dead, but not by my hand!"" However, we do meet a vigorous John of Gaunt and Harry (later Henry V) displays a forthright devotion to Richard: ""He piles wrong on wrong. . . . But I love him, still or loud!"" John is the ever-faithful servant to the realm and brother Harry, methodically going about his duties as pacifier and governor of the Scottish Borderland, acting as all-around confidant and moderator. This is a generally unaccented account of the events of the period--the fall of contentious nobles, uprisings quelled in Wales and Scotland, dealings with France and Rome, the spread of the Lollards, etc. A reasonable reading of the times, but one wishes for less stately converse, more consolidation of material into the dramatic confrontations which were Heyer's forte. Even the inclusion of such marvelous Middle English words as ""orgulous"" (""over-proud,"" the glossary tells us) doesn't serve to move matters along, but predictably this is more respectable in scope and scholarly responsibility than most, and will not diminish the luster of Miss Heyer's perdurable happy gestes.