The Hammer of Thor by Rick Riordan

Magnus Chase has settled into his life in Valhalla, as well as anyone can settle into a life, or afterlife, of an einjher.  Then Thor loses his hammer, the one he rules the world with, the one that keeps the earth giants under control, you know, the important one.
The second novel in the Gods of Asgard series is very good.  Funny, fast paced and has a wealth of information about the Norse gods and goddesses.  Highly recommended and not only for the young adult reader.