Iron Sunrise (Eschaton, #2) by Charles Stross

When the planet of Moscow was annihilated, its few survivors launched a counter-attack against the most likely culprit: the neighboring system of 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.

And the one person who knows is a disaffected teenager who calls herself Wednesday Shadowmist. But Wednesday has no idea what she knows…