At the beginning Miyuki warns us anticipating readers that is not a cheat book or a guide- much to my disappointment, I jumped in with an open mind and was indeed- pleasantly surprises.
Team Ico was clever at not giving us any back story as to why kids ago grew horns were sacrificed to The Castle In the Mist, which added mystery to the game with lots of un answered questions and many possibilities to what Ico's and Yorda's future would be.
So in this book- the story took place after Ico is born with the curse, now im not going to say much more about the big WHY, since that would be a huge spoiler.
Miyuki also adds more history to the castle itself, before the castle was cursed and when dwellers were living there - it is apparent there is more of a sad story between Yorda and a man she once knew, Miyuki introduced lots of new characters to the book that are not related to the game at all since Team Ico allowed her full rein of the story.
However most of the dialogue and description of Ico's surroundings are accurate, so it wasn't a total disaster, bearing in mind the author is in no way any part of the games developemeant at all.
It was fascinating reading a story about s game I have such good memories over these few years of my childhood and to get more close with the characters and feel more for them.