Day of the Giants, by Lester del Rey

Leif Svensen is kidnapped by Loki and taken to Asgard where he's put to work designing weapons for the Aesir to use against the giants when Ragnarok comes. With the help of clever dwarfs, Leif produces gunpowder grenades, explosive-tipped arrows, and fission bombs. Oh yeah, the dwarfs can refine U-235, don'tcha know?

It's books like these that reveal my almost-complete ignorance about Norse mythology. I'm sure there was a lot I missed out on.

del Rey writes well and Day of the Giants is engaging. I'm labeling this science fiction because it feels like science fiction, even though there's magic and this is clearly fantastic.