I love it when Diana Wynne Jones makes you think you're reading a certain type of story until BAM! Some choice tidbit clues you in to another reality altogether and you've gotta twist everything you know at least a quarter turn to the left so it all fits.
She does it very well in POWER OF THREE. As an added bonus, this children's fantasy published in 1976 is deeply concerned with issues like water rights. Hello, topical fiction.