I loved it. 
 At the beginning I had some trouble getting into the story. I don't really like when in a book someone tells something that happened before (kinda like in Wuthering Heights). I find myself always waiting for something to happen in the present. I also had some problems understanding everything, it was an English more complicated than usual. 
 But then, I loved the way Maddie and Julie met, and the way it was told. I loved their relationship. I loved Elizabeth Wein's way of writing the story, slowly discovering things in the past, and in the present. I really felt like I was reading something real, even more because I know a lot of places named in the book ; for example Natzwiller-Struthof, I was there more than once, I was in the room where there used to be "medical experiments". 
 And finally, it was Maddie's turn to write. AND THE PLOT TWIST. And everything after that. I just lost it.