Runelight by Joanne Harris
Five past midnight, three years after the End of the World, and, as usual, there was nothing to be seen or heard in the catacombs - except, of course , for the rats and (if you believed in them) the ghosts of the dead. Runelight is set in a world coming back to life, but it is also a world in conflict. For Maddy, born with the mark of magic on her hand, the return of the old gods and the overthrow of the brutal Order who suppressed them is a time of excitement. But for Maggie, raised as part of the Order, it is a time of chaos and desolation, as her family and values are wiped out. Like the two girls, the new world remains divided - the old regime has gone but with nothing to take its place, anarchy is beginning to spread. Joanne Harris draws us back into the richly imagined world first introduced in Runemarks - a world not unlike our own, had it been shaped by the Vikings instead of the Romans. But if its inhabitants, including Maddy and Maggie, can't solve their differences soon, they will destroy their world altogether.