I would mark it as young adult for some rather hot sex scenes.
I've read it as a diary of a young troubled woman. It was interesting and well written and I enjoyed her story. I felt sad for her and understood her issues.
I also had some problems with it.
I felt it was a little naive. My main problem was the fact that I felt like reading about Bella in the role of Ana. She was rich, but didn't behave like one; he was rich and behaving like one. I don't know, but I don't think that people their age act that way. It would have been better if they were older. It would have given the story a different depth.
All in all, I enjoyed this book, but still the YA staff is not my thing.