The Hammer of Thor by Rick Riordan

"Magnus Chase, you nearly started Ragnarok. What are you going to do next?"

It's been six weeks since Magnus and his friends returned from defeating Fenris Wolf and the fire giants. Magnus has adjusted to life at the Hotel Valhalla—as much as a once-homeless and previously alive kid can. As a son of Frey, the god of summer, fertility, and health, Magnus doesn't exactly fit in with the rest of Odin's chosen warriors, but he has a few good peeps among his hallmates on floor nineteen, and he's been dutifully training for Ragnarok along with everyone else. His days have settled into a new kind of normal.

But Magnus should have known there's no such thing as normal in the Nine Worlds. His friends Hearthstone and Blitzen have disappeared. A new hallmate is creating chaos. According to a very nervous goat, a certain object belonging to Thor is still missing, and the thunder god's enemies will stop at nothing to gain control of it.

Time to summon Jack, the Sword of Summer, and take action. Too bad the only action Jack seems to be interested in is dates with other magical weapons. . . .