Lucifer's Hammer — Larry Niven; Jerry Pournelle

THE LUCKY ONES WENT FIRST...
The gigantic comet had slammed into Earth, forging earthquakes a thousand times too powerful to measure on the Richter scale, tidal waves thousands of feet high.

Cities were turned into oceans; oceans turned into steam. It was the beginning of a new Ice Age and the end of civilization.

But for the terrified men and women chance had saved, it was also the dawn of a new struggle for survival--a struggle more dangerous and challenging than any they had ever known....