I struggled to get started with the book (it took me 9 months to finish!) and I'd put it down for a long time, but I picked it back up because my mom had read it and really liked it and I hate not finishing a book. At some point, I became a little obsessed. 
 I think what changed it was the move from set up and explanation as to what happened to the actual action. Cassie on her own was boring. Her memories and the set up to why she was looking for Sammy was just sort of there. The addition of other characters, like Cassie meeting Evan and the introduction of Zombie and the child soldiers and those parallel story lines helped to make the book move "faster." In the end, the fact that it took me so long to get into it kept me from giving it a rating that was any higher.