Iron Sunrise by Charles Stross
When the planet of New Moscow was brutally destroyed, its few survivors launched a counter-attack against the most likely culprit: the neighbouring system of trade rival New Dresden. But New Dresden wasn't responsible, and as the deadly missiles approach their target, Rachel Mansour, agent for the interests of Old Earth, is assigned to find out who was.
The one person who does know is a disaffected teenager who calls herself Wednesday Shadowmist. But Wednesday has no idea where she might be hiding this significant information. Time is limited and if Rachel can't resolve this mystery it will mean annihilation of an entire world.