THE DEATH OF NAPOLEON By Simon Leys (Picador, $77) WITH so many tantalising what ifs in history, it is hardly surprising that they are sometimes turned into fiction. The inability to believe in the passing of 20th century folk heroes like Elvis, must also apply to the previous century when the heroes of the day tended to be soldiers and political leaders or, in the case of the Emperor Napoleon, both. So what if he had not really died during his exile on the island of St Helena? Simon Leys takes up the story. In his version of events, rightly nailed as ''a small masterpiece'' by a reviewer in Le Monde, the great man managed to escape from the island and left a double to take his place. The double unobligingly kicked the bucket just as the former Emperor slipped back into France to make preparations for his restoration to glory. Officially dead, how could Napoleon rally his followers to plan the restoration? Indeed how could he prove that it is he who is the authentic version and the deceased impostor, who is the fake? I would not like to spoil any potential reader's pleasure by answering these questions but will hint at some of the problems by saying that Mr Leys, at one stage, has the great Bonaparte organising a successful fruit distribution business among a band ofdie-hard Bonapartists who, with one significant exception, seem unaware of the true identity of their organiser. Many of those Napoleon encounters in his escape from St Helena recognise something special about him, without quite knowing what it is. The greatly-despised Nigger-Nicholas, the cook on one of the one vessels smuggling the incognito Napoleon towards France, senses something about the strange lowly crew member who joins the voyage. His kindness to the unresponsive Napoleon is a source of mystery to the rest of the crew but provides the first hint of the magnetism still exercised by the former ruler of half of Europe. Mr Leys understates everything about his subject with tremendous skill. The irony of his circumstances is seen far more clearly through the author's use of inference rather than direct explanation. He takes the reader on Napoleon's journey back to France in a manner which makes you wait anxiously for every encounter, every pitfall and even every triumph of a man whom many historians have treated with less than reverence. At one point in the journeyhe joins a group of British tourists on a visit to the former battle grounds at Waterloo, falling into the hands of an old charlatan guide who, in the best traditions of theatrical farce, furnishes him with an account of meeting the Emperor Napoleon. Lamentably, Mr Leys does not address himself to the subject of Bonaparte's relationship with ''not tonight'' Josephine. But this is a trifling quibble about a tightly-constructed and exquisitely executed book, written in French but rendered with skill inEnglish, thanks to the translation by the author and Patricia Clancy. Alas, The Death of Napoleon is a mere 100 or so pages of, it has to be said, unusually large type. In reality, it is a short story, but one which would not benefit from being prolonged. On the contrary the skill of the author lies in telling his tale so superbly within these very limiting parameters. Simon Leys, the pen name for Pierre Ryckmans, is probably better known in Hong Kong as one of today's most-perceptive sinologists who correctly identified the circumstances building up to the Tiananmen Square massacre. He has consistently provided some of the most trenchant comment on events in China, is also an art historian and is clearly a formidable writer of fiction.