Wow, everything sucks about this book.
The writing is very fast and it expects you to be familiar with the game, so if you aren't... you'll be left confused. That being said it has be written for kids, BUT the way the story jumps around it's just not good writing. Even for children. Like we found someone, then they are running off and we are running off, then we catch up, then someone appears the disappears. It's just that quick and the boss fights last as almost as long as a summary paragraph.
Then there's the story...
This is very much a damsel in distress story. All the women in the story are just there as objects. In this case their hearts are needed to open and door. You could replace them with a magic gem and the story wouldn't be affected. This extends to Kairi too, who you barely get to hear about before she disappears and then is rendered a lifeless doll for Riku to cart around.
After she is saved by Sora, she is then told "IT'S TOO DANGEROUS FOR A GIRL" and left behind. OF COURSE. We save her then ditch her like the magical gem she is.
Not only that in the course of the story Sora shows some sympathy for Captain Hook after he is told how hook lost his hand to the crocodile. Donald Duck dismisses his sympathy, because Hook "Took Kairi and Wendy prisoner." I'm sorry the two don't equal one another, and Hook's hand happened in the past.
So basically since this book is aimed at boys we are teaching them that women are objects and to be unsympathetic, emotionless monsters. :|