are you a "real" person?

I read a book in sixth grade about a girl who thought that she was the only real person in the world. Allow me to elaborate. She was convinced that people only existed when they were around her and that her life was the only one that was constantly present. So she thought that when she left the room, it and the people in it would disappear into darkness. Her theory also included the idea that when these other people were “in darkness” they were being programmed to think that things had actually taken place in their lives and to have memories and such. But then she meets this really awesome friend and decides by the end of the book that her presumptions were wrong, or at least that this friend of hers is a “real” person too.
Though I assure you the book was entirely juvenile and indeed targeted towards a sixth-grade level of comprehension, it still presented an interesting idea. Have you ever wondered what someone is doing at a precise moment in time? There’s no way of knowing without asking them, or setting up video cameras in their house, which is illegal and kind of weird. Or have you wondered what a celebrity does with their days. For instance, what is Miley Cyrus doing right at this very moment? Moreover, when you leave your room, does it remain exactly the same as when you left it? When I was little I thought that my dolls would get up and dance around my room whenever I left it like in Toy Story...but really. It’s hard to imagine what happens when you’re not around. And since it’s impossible to be in two places at once, it’s impossible to see everything all the time. It’s a bizarre thing to think about, but there it is. I don’t even remember the name of the book, much less the author, but I’ve kept the meaning and the story of it with me for years now, and I’ll think about it from time to time.