Like the movie, the book tells of a young girl draw into a fantasy world by her own overactive imagination in order to save her little brother, who has been stolen by the goblin king, who says he is only seeking favor in her eyes, and seems to have fallen in love with her.
The book follows the storyline of the movie exactly, but offers more insight into the characters thoughts and actions. I can remember in particular that the ballroom scene was quite staggeringly more descriptive. A wonderful book.