I didn't like this one quite as much as the first one, The Sea of Trolls, because it didn't seem to be as original as that one (I'd never read anything about Norse berserkers before!) but it was still fun. Jack is back home in Saxon England, but his sister is not behaving nicely at all, she's a very spoiled brat, and so their father takes them to the local monastery to have her exorcised, fearing that she's possessed. Well, it turns out she's a changeling--Jack's real sister was kidnapped while a baby and this elf child put in her place! So off Jack goes to the underground Elfland to try to find his sister and bring her back, braving magical mushrooms and the sparkling illusions of the elves. His traveling companions are the freed slave Pega and the Roman slave Brutus--who is more than he seems. And the Viking warrior girl Thorgil soon shows up, too. Plus there are some fun "pookas" or hobgoblins (they go by many names), Bugaboo and his Nemesis, who are a hoot. Kind of a comedy duo of short proportions. They were the best thing about the book. Beware, it's book two of a trilogy and has a lame ending, in my opinion.