NAPOLEON 1812 by Nigel Nicolson (Harper & Row: $16.95; illustrated). Napoleon endlessly fascinates. For nearly two centuries, historians, amateur and professional, have recounted the emperor's rise and fall, his larger-than-life ambitions, his obsession with Josephine, the governments he created, the popes and kings he subdued. But more than anything else, his military exploits continue to captivate historians and readers alike. Nigel Nicolson's crisp new account of Napoleon's disastrous 1812 assault against the czar's Russia adds little to what we already know. But for those who wish to revisit the horror and the folly of this distinctly inglorious campaign, Nicolson provides a lively and engaging narrative. His information comes largely from the memoirs of those who fought with the emperor, and while these documents heighten our sense of the battles' mammoth tragedy, Nicolson tends to take them too literally. As a result, we view the events too much through the eyes of generals more interested in looking good to posterity than in strict historical accuracy. Still, Gen. Caulaincourt's observations of Napoleon's character and behavior remain invaluable. For Caulaincourt showed how Napoleon's legendary self-confidence, distorted now by autocratic power, had blinded him to the realities of mother Russia: climate, distance and the peasants' ferocious brand of patriotism. But as emperor and commander of the Grand Army, Napoleon had his way. And the outcome proved far more devastating than even the pessimists had foreseen: Napoleon entered Russia with half a million men; he left it with 25,000. By invading Russia, he lost his army and soon his throne.