This may not be a great literary achievement, but what it does it does well. 

The question of what happened to Amber to land her in a coma, is exciting and I liked that the narrator (Amber) doesn't know either. The three story lines, 'now' - Amber in a coma; 'Then' - the weeks leading up to the event that caused the coma; and 'before' - the diary of a 10 year girl about 25 years previously, complement each other well. The story is exciting and entertaining and you never know quite what is real and what is not. Ok, the solutions in the end is somewhat far fetched, but that is most often the case, so it doesn't bother me too much.