At first I wasn't sure about this, but I'm so glad I continued reading. The mystery of who took Dan kept me guessing the whole time, and whenever I thought I knew for sure who did it something else would happen. The story was so great and I loved the ending - it was perfect.
I think the only thing that bothered me about the book was the writing style. By that I mostly mean dialogue - sometimes it just didn't flow or feel like what people would actually say.
Overall though it was fantastic, and the fact that iBooks had it for free just made it that much better!