The Fallen Moon

From the front flap of the Gollancz first edition: "Kaleva is Earth's first and only interstellar colony, discovered by Lucky Sariola who was transported there by an Ukko, a mysterious asteroid-like entity that responds to stories told to it - in Lucky's case, those of her Finnish grandmother. Centuries later Kaleva, a planet of forest, lake and sea, is a quasi-feudal society salted with advanced technology and inhabited by humans, but also by the Isi, snake-like aliens, and their servants, the humanoid Juttahat. Now Queen Lucky, half mad and newly widowed, is obsessed with finding the Ukko that brought her to Kaleva, for it is her other half, her 'sister', and by meeting her she believes she can feel complete. But this obsession is potentially disastrous, for the Isi too are looking for the Ukko as part of their design to enslave the humans. Understanding this, one of Lucky's daughters (and with obsessions of her own) crowns herself rival queen. As summer turns into unseasonable inter (brought about by Lucky's mutant grandson Demon Jack) and elysian peace turns to bitter war, the Ukko, once more, has a role to play in the history of Kaleva..."