Darwinia

Darwinia, Robert Charles Wilson, Tor, 1998

One day in 1912, Europe disappears. It is replaced by thick jungle, full of plants and animals not native to earth.

Several years later, a major expedition is planned up the Rhine river as far as possible by boat, and then by foot to the Alps. Guilford Law, a young American, joins the expedition as official photographer for the National Geographic Society. Many months later, after everyone has given up on the expedition, and Guilford's wife has moved to Australia, thinking him dead, Guilford emerges from the jungle as the only survivor of the expedition.

Guilford meets a ghost/double of himself in an army uniform with tales of another earth where Europe has just finished a world war. The double tells Guilford of a galactic-scale Archive of all the intelligent civilizations that is under attack from within. Everyone in the new Europe, called Darwinia, has been enlisted in battle on one side or the other.

This one is in a class by itself. If I could, I would give it three thumbs up. I thoroughly enjoyed this superior piece of writing.