Thor's Hammer — Will Shetterly

The Norse gods are in an uproar--Thor's hammer has been stolen! The goddess Freyja has foreseen that a human boy will help find the hammer; the Basset sails into the mortal world to find him. But when the Basset returns, it carries not one boy, but four--all of whom must change if they are to fulfill Freyja's prophecy and save the gods from destruction.