Nation by Terry Pratchett
Nation is set in a world very much like ours, but with some subtle differences. There is an England, for example, but there is also a “Re-United States”, and “the Great Southern Pelagic Ocean”.
At the start of the book, England has been hard-hit by the Russian Influenza (the exact date is a little hard to pin down. There are ships, and they seem to be sail, but that doesn’t narrow things down much. There is a British Empire. Clothing would help give a rough idea, but it isn’t described in any detail (and we spend hardly any time in England, anyway). Speech patterns suggest to me the 19th century, early-to-mid) and the King, along with most of the Royal Family, is dead. The heir must be located immediately, but the heir, having previously had 138 people between him and the throne, has taken a governorship in the above mentioned southern Pelagic Ocean. He recently called his daughter to join him.
Meanwhile, in the Southern Pelagic Ocean, Mau has left his Nation to spend a month on the island of the boys, after which he comes home a man. Well, technically, he’s still a boy until there is a ceremony, but he is on his way home when a giant tsunami sweeps over the entire area, burying boy’s island completely in the sea. Fortunately, Mau was already off the island and heading for home, in deep water when it struck, so he survives. He is pleased to remember that his home island is bigger and based on a mountain, so it was unlikely to have been over-run by the wave; he is disconcerted to remember that everyone was likely to have been down on the beach, waiting for him to come back as a new man. And yes, when he arrives home, the island is still there and the village is intact, but all the people are gone.
Poor Mau seems likely to go crazy, moping around his island, being yelled at by his ancestors for not performing ceremonies he never learned about, and being haunted by his personal ghosts, but it turns out that one of his ghosts isn’t a ghost at all (though she’s so pale he can be forgiven for mistaking her for one, and calling her “Ghost Girl” after that), but an English girl named Ermintrude Fanshawe, though she prefers to be called “Daphne” (wouldn’t you?). It seems she was shipwrecked on the island–literally, her ship having been driven quite a ways inland by the wave. She was the sole survivor.
Soon other people start showing up, survivors from other islands that were not so lucky in terms of mountains. They expect to find the Nation here and are a bit disappointed to find only Mau and Daphne. Mau should technically be the leader, being as it’s his Nation’s island, but he’s also technically just a boy. Daphne is not very adept in being a woman in the terms of the Nation, though she’s certainly willing to give it a go (it’s more interesting than things were at home, at least until the voyage).
And, just to complicate things, somewhere out there is a nation of cannibals, of whom everyone has heard but who no one has seen for a long, long time. Maybe they were destroyed by the wave. Or maybe they’re heading in to investigate the damage done by the wave . . .
Like a lot of the best YA fiction, this is about growing up. Both individual growing up and, because it’s Pratchett, growing up as a, well, Nation. Both Daphne and Mau read like real people, and neither culture is treated as silly (some parts of English culture are seen as inappropriate for life on a tropical island, though), though some people are just wrong (Daphne’s grandmother and First Mate Cox both are, but I think that’s because, to differing degrees, they treat people as things–which seems to be the biggest sin Pratchett can imagine).