Nation is a book published by Terry Pratchett in 2008, and is the first non-discworld work by Terry Pratchett since the Johnny Maxwell trilogy of the 1990s. The book takes place on a (slightly alternative) earth in the 1860s.
The book takes place on an island called "Nation", somewhere in (what we would call) the South Pacific. The island is considered significant and large by its inhabitants, although it seems to be no more than a few dozen square miles in area. The plot of the book involves a tsunami that wipes out almost all but one member of the Nation, as well as people on surrounding islands. The only survivor of the Nation is Mau, a boy who has just completed his manhood ritual on a distant island, and is in a canoe when the wave hits. Also on the island is a British naval ship, with only one survivor: Daphne, the daughter of the British governor of the area. Together, Daphne and Mau must quickly overcome the remaining natural hazards and cultural barriers if they are to survive in this world. Other survivors join them, and through a series of plot complications, different philosophical and cultural issues are brought up and then (such as it were) solved.
Although not a Discworld novel, the book explores many of the same themes that have been important in the later Discworld series: how people's individual needs, thoughts and decisions can be both constrained by, and build, a society. Also, compared to the Discworld novels, it tends to to be more somber in tone, having less word play and witticism, and more "realistic" dialog and description. It is still recognizably a Pratchett book, however.
Terry Pratchett's fans don't need a recommendation to read any of his books, but this book would actually be one of the first books I would recommend to get someone reading Pratchett, in part because the entire history of the Discworld is not needed to read it.