80AD - The Hammer of Thor — Aiki Flinthart

Book 2 of the 80AD series sees Jade, Phoenix and their companions flung into the frozen wastes of northern Europe. Here they must somehow reach Asgard, home of the Norse Gods, in order to steal the Hammer of Thor. They are far from civilisation, snowbound, hunted, lost and on a deadline. If they don't reach Asgard in time, bad things will happen: Ragnarok - the end of the world. Unending winter, followed by hellfire and an almighty battle between the gods and giants. If this world is destroyed, Phoenix and Jade die with it and their bodies, back in their own world, die as well. Standing in their way are trolls, wolves, their old enemy Feng Zhudai and a new one - Loki, god of mischief and chaos. To save this world and themselves, they must get to Asgard in time. Of course, it would help if they knew where it was. Then, somehow, when they do find it, they have to steal Thor's Hammer - and everyone knows that stealing things from Gods is a bad idea.