Nightfall by Isaac Asimov and Robert Silverberg.   These two renowned writers have invented a world not unlike our own--a world on the edge of chaos, torn between the madness of religious fanaticism and the stubborn denial of scientists. Only a handful of people on the planet Lagash are prepared to face the truth--that their six suns are setting all at once for the first time in 2,000 years, signaling the end of civilization.