Yes, I noticed that as well. As I started reading I thought the book should have been in the fiction section. But as I got further into the book, I realized that this couldn't be classified as a historical novel either. The way it is written, some of the entries use dialogue to tell the story, others use the narrative. It really IS written as a journal. So, I suggest you leave it as it is. I really loved the book!
I bought the book thinking that it was a biography, and discovered upon reading it that it was written in the first person. It makes no difference. The historical facts are well-researched and footnoted, and the book was very enjoyable.