The Fallen Moon. Mana Book 2

The second and concluding volume of Ian Watson's extraordinary epic, The Book of Mana. Kaleva is Earth's first and only interstellar colony, discovered by Lucky Sariola who was transported there by an Ukko, a mysterious asteriod-like entity that responds to stories told to it - in Lucky's case, those of her Finnish grandmother. Now Queen Lucky, half-mad and newly widowed, is obsessed by relocating that Ukko - but this is potentially disastrous, as the snakelike alien Isi are also on its trail as part of their design to enslave humans.
Understanding this, one of Lucky's daughters (with obsessions of her own) crowns herself rival queen. A summer turns into unseasonable winter and elysian peace turns to bitter civil war and Ukko, once more, has a role to play in the history of Kaleva.