Joe Haldeman's Old Twentieth is a paean to the 20th century, although it is actually set in the future. In the 21st century a way is discovered of making humans virtually immortal, though a self-repairing process. This prompts a war that kills off 7 billion humans and leaves fewer than a million immortals. But eventually the survivors build the world back up and even send a group of these immortal on a thousand-year journey to the stars.

(Source: Ace Books)