How fascinating. Must be amazing watching her mind develop!
I was an early reader. I was also an early riser. My parents told me to read a book, and not wake them up. So I did. Prolifically.
We had all kinds of books in the house, from encyclopedias to sci fi to "kids" books, to classics.
I have no idea why I picked up one book over another, and don't actually remember what I read when I was a kid (other than "Are you my mother?" and Shel Silverstein), but the important thing is that I read.
When I got to school, I was far more prepared for English than my peers. I rocketed to the top of the class, and was in advanced english courses before I left junior high school.
Today, I make an excellent living as a writer/communicator.
So here is my totally unskilled suggestion: Provide the widest range of reading material possible. Let her mind drive what she picks up when her mind is ready for more. I don't know that guiding her consumption 100% is the best way to go... serendipity has to have a place.
I never read "at my level." I was always capable of reading and understanding books that were far beyond my supposed-level.
Give her range. That's my suggestion.