Jacob has been going through the mirror for years, and is now a man in his twenties. I think this is where people get confused as to the target audience; as though kids can't read about a twenty-something going on a quest. Anyway, Will follows Jacob through the mirror, is attacked by the Goyl, and thanks to a curse of the Dark Fairy (mistress of the king of the Goyls), is slowly turning into a Jade Goyl. Humans are becoming Goyls and joining the dark forces. There are legends in the Goyl world about the jade bodyguard, who will keep the King from ever being killed. Jacob is racing against time and the Goyl Army to save his brother before all of his humanity is gone, and he becomes a Goyl.
This book was really great. There are sketches throughout the novel, which adds to the fairy tale aspect of the story. It really is a fairy tale within a fairy tale; Jacob makes a living in this other world by finding and selling magical artifacts: a piece of Rapunzel's hair, a rose that puts you to sleep and keeps you from aging, and fairies galore. Cornelia Funke has created an amazing world--I think she's a fabulous writer.
So don't hesitate to read this novel. Who cares what age it's meant for? It's a well written, imaginative fairy tale that keeps you turning the pages, until the last sentence. Can't wait for the next one!