Magnus Chase and The Hammer of Thor

The second book in this number one, bestselling series from the creator of Percy Jackson. Thor's hammer is missing again. The thunder god has a habit of misplacing his weapon - which happens to be the mightiest force in the Nine Worlds. But this time the hammer isn't just lost, it has fallen into enemy hands. Magnus Chase and his friends must find it, fast. If they don't, the mortal worlds will be defenceless against an onslaught of giants, and the Nine Worlds will burn. But the only person who can help them is the gods' worst enemy, Loki - and the price he wants is very high.