Nation by Terry Pratchett

Although Nation is listed as a "young adult" novel, I thoroughly enjoyed it. A giant tidal wave destroy much, including all but one boy on the brink of adulthood on an island. A young girl from around the world is the sole survivor of a shipwreck caused by the same tidal wave. As they work out a means to cooperate and get what they need, other survivors slowly arrive.

Nation is a parallel universe story, where lots of things are the same as they are here but not quite everything. The little twists that are different are often quite amusing and also thought provoking. It's a lovely story with great characters and setting, great backstory and a fantastic climax. Highly recommended no how old you are, as long as you like a good story.