I was a much stranger 13 year old than that, and look how I turned out...
What?

Oh, ok, fine, she's acting a little weird, but at least she's reading. And yes, like it was said before, she's got her own special place... It sounds like something as harmless as an imaginary friend, but thriteen is a little old for that. At the same time, I think we can all look back at that sort of age and think "jesus!", it's an embarrassing age, and it's very possible that in a week she won't even acknowledge that she said that to you.
And what's the worst case senario? She's a little batty and has an avid interest in books. Which you could easily classify alot of people as.
But have you considered that the books are reading her thoughts? That her thoughts appear to be translated in front of her on the page... and that she took this as a literal sign from God/supernatural power or whatever. I experimented with my spirituality in not entirely similar but just as weird ways when i was around 12/13, but I certainly wouldn't have gone up to my big sister and told them the details, because they'd look at me funny and then treat me like a leper, which, perhaps not worded as strongly as I put it, is exactly how you've reacted. Instead, why don't you play along and get the
details. Also, remember that being a child and being completely insane is very similar in terms of symptoms. Little kids are batty like that.