I thought McKillip only had one scifi book – but it’s actually three. Moon Flash (and its sequel, which I have not yet read) could be mistaken for fantasy at the beginning, but turn out to be scifi as the story moves along.
Kyreol is the daughter of the village Healer, in a simple land ruled by the River. At the end of the River is the Face, and the Moon Flash happens there once a month, governing all that the River people do.
Kyreol’s mother disappeared long ago, but she and her father do not believe her dead. When Kyreol is betrothed and moves to her bethrothed’s house, she’s suddenly seized b y the urge to follow in her mother’s footsteps, to see what lies beyond the end of the river.
What she finds is that the river doesn’t end, only flows through many other lands, and at the end, she may just find her mother. (That’s where I’ll stop – half the fun of this book is trying to figure out exactly what really is going on – the author holds that very close, throughout the course of the story.)