I think I want to give this book more of a 2.5 star rating. I have mixed feelings about the book. On one hand, I loved reading this. On the other, I admit that the writing still was kind of, "Oh, well, thanks for telling me." So, let's break this down into categories.
My general reactions about reading: 4-stars
As I said before, I loved reading this book. I cannot tell you why, but something about it pulled me in and made me turn page after page. I was particularly interested in the real people that the characters were based on, so I frequently interrupted my reading to do some history research. I really like how the author included sources for readers to find more information. That was very kind of her. I may have to check out some of those books she recommended.
Daniel: 0.5-star
Here I am specifically signally out Daniel for a reason. To me, he seems whitewashed. The only reason I know he is Jewish is because he says, "I'm a Jew." I don't see any other references to Judaism or Jewish culture when Daniel is in the picture. It seems like someone who claims, "I'm Italian!" when he or she grew up in America, his or her family has been here for many generations, and the person follows more of an American culture than Italian. The person may have Italian genes, but culturally, the person is American. My point is...Daniel doesn't seem Jewish to me. Maybe that is the author's point, to make Daniel seem like any other German, but I wish there were more references to Jewish culture.
Love story: 1-star
If you like cheesy romance stories, then this love story deserves 5-stars. I do like the development of Gretchen and Daniel's relationship, but when they are actually together and actually in love...it is just too much. Their relationship starts off rocky and eventually grows a little over time, but one day she looks at him and says, "I'M TOTALLY IN LOVE WITH YOU IN EVERY WHICH WAY." Then I felt completely swallowed by a landslide of cheese. It is a very idealistic romance. It's not necessarily wrong; it's just not my preference.

Action (or things happening!): 5-stars
The plot keeps moving. Some things that happen make me think...WTF, but otherwise the scenes move pretty fast, and I like the pace.
Writing style: 1-star
I don't want to insult the author, but sometimes (by which, I mean a lot of the time) the writing does a lot of telling. This actually increases the pace of the book. However, sometimes it's nice to have a little bit more creative description of what's going on. I'm paraphrasing here, but basically a scene goes, "She turned around, and the general spun his gun in an arc. Then she ran." We hear a lot of her heartbeat. Poor heart...it must be extremely overworked. Some details are repeated frequently, and the way scenes are depicted seems limited. I realize writing is very hard to do, so I don't want to be too harsh. After all, I've never published a book, but as a reader, I expected a little bit more, especially given the ages of the characters, which reminds me.
Gretchen: 3-stars
I think Gretchen is a strong heroine because she tries her best to get involved. She may not do it in the smartest way, but I think she's also trying to find her path, which she may not exactly know at the end of the book. Well, she really doesn't know. That's why there's another book coming! Moving on, the thing that bothers me about Gretchen is that she acts young for her age. If I had to guess her age based on the book, I'd say she's around 14, but Gretchen is actually around 18-20, I believe. I get accused of acting like a kid a lot, so I can understand a character acting "young," but everything about Gretchen pretty much seems like a young teen instead of an older teen.
I don't think it's fair for me to rate this last bit, but the psychology kind of made me rolled my eyes. I know the author used the type of psychology that they used in book's historical time period, but it's definitely not the same in modern times anymore.
Overall, I'd like to say that even though I really enjoyed this book, based on the way I rated other books, I feel compelled to rate this one as a 2.5 book, mostly because of the simple writing style. However! I really am looking forward to the next book. I want to know what happens in Gretchen's future. I really hope she becomes a doctor.