Day of the Giants

Then, on his way to find his madcap twin, Leif met a stranger who knew his name and who spoke of "the Fimbulwinter already upon us." Fimbulwinter! The dreadful winter that in Norse mythology preceded Ragnarok -- the final war between the gods and the giants! Fimbulwinter -- which presaged the Day of the Giants!

Leif Svensen and his brother were caught up in the destinies of a real but alien world. For if the giants triumphed, they would overrun Earth; and if the Aesir -- the gods -- won, Earth would be their footstool!