The Aftermath by Samuel C. Florman
When a comet destroys almost all life on Earth in the year 2010, a shipload of engineers, stranded along the South African coast with a small surviving community, must begin the task of starting over. Though the tiny region of Kwa-Zulu Natal is spared from total destruction, its inhabitants have been cast back, physically, to the Stone Age. They must find a way to rebuild from scratch the world as they knew it, but in an unpredictable, and sometimes hostile, environment, survival in itself is difficult.