Of all the figures in Norse mythology, none proves more slippery to pin down than Loki. He is neither entirely divine nor wholly monstrous, neither reliable friend nor consistent enemy. This shape-shifting trickster weaves through the old stories as both helper and saboteur, his clever schemes sometimes saving the gods and sometimes bringing catastrophe upon them. His complexity has fascinated audiences for over a thousand years, from medieval Scandinavians gathered around hearth fires to modern cinemagoers watching Marvel blockbusters. Loki refuses simple classification, and that ambiguity is precisely what makes him so compelling...