ByMerle RubinMerle Rubin reviews books regularly for the Monitor. Setting out to write the official biography of George V in 1949, Sir Harold Nicolson commented on the difficulties of describing his subject's life before he became King: ''He is all right as a gay young midshipman. He may be all right as a wise old King. But the intervening period when he was Duke of York . . . is hard . . . to swallow. For 17 years, in fact, he did nothing at all but kill animals and stick in stamps.'' While Kenneth Rose gives King George's twin passions of hunting and philately their due, he wisely wastes little time on the King's early years. By Page 74 of this 500-page biography, the 44-year-old King is already on his throne. ''King George V,'' winner in England of the Whitbread Award for Biography and the Wolfson Award for History, is proof positive that a skilled biographer can write a fascinating book about a boring person. Even his staunchest defenders could not claim that George was very interesting: only, perhaps, that neither he nor his wife, Queen Mary, was really as stiff and ordinary as was commonly suspected. Having already lived half his life during the reign of his grandmother, Queen Victoria, George came to the throne in 1910. What many of us now think of as the ''modern'' age was ushered in by a series of cataclysms, the largest of which was the virtual destruction of an entire generation of young men in World War I. Rose shows us the King's agony at this slaughter, rendered all the more acute by his absolute powerlessness to stop or mitigate it. But this biography does not spare us the King's obtuse attachment to such generals as Douglas Haig. Haig's lack of vision would have far more devastating consequences than any blindness of the King's. George disliked change of any kind on general principles, so it is not surprising that he deplored the Russian Revolution, which had, in addition to bringing about plenty of change, murdered the Czar and Czarina, each a first cousin of his. Rose refutes once and for all, using unimpeachable sources, the legend fostered by the late Lord Mountbatten and others that Lloyd George's government had overruled the King's desire to rescue the Czar and Czarina. In fact, fearful of republican, anti-monarchist sentiment in his own country, the King dissuaded the government from granting Nicholas, Alexandra, and their family political asylum. Despite his tendency to act as an apologist for a King as dogmatic as he was narrow-minded, Rose provides a believable portrait of a staunch patriot and a fundamentally honest man. Rose has been especially skillful in his use of primary sources, mining the papers of countless statesmen, diplomats, and courtiers. His well-organized narrative, written with great elan, is as much a history of Great Britain as it is a portrait of the King.