I have read this again as an adult. The last couple times I was around Harriet's age. It is a lot different from what I remember, and I am highly influenced by what I remember in the movie as I've seen that several times in my adult years. I feel like it still holds up, but honestly maybe not a glowing 5 star rating. It isn't a perfect book by any means. I'm leaving it at 5 stars, because this book (and the movie) were very important to me when I read/saw them for the first time at age 12. I even kept a journal and thought I was a spy for a while.
As an adult, I can say that Harriet was a little bit of a bratty, spoiled child, but I hardly blame her, because of how she was raised.
I also want to say that if Janie was really her best friend, I don't believe she would have read her diary/journal, and should have gave it back to Harriet. None of the mess would have happened. Harriet could have apologized for hurting people's feelings, but I don't think she should have to lie, maybe little white lies. Her journal was her personal thoughts and something she really needed. There are people who believe she might have been on the autism spectrum and the journal was her grounding activity.
Also how Janie and Sport treated Harriet does not come off how a best friend would treat another friend, even if they were angry at each other. I might be able to believe the other classmates actions, because they were never that close to Harriet. Some people think Golly was a bad influence, but I don't think so. She encouraged Harriet's imagination and seemed like the only person who could truly understand her, but that is just my opinion.