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.
I was a struggling reader until the 3rd grade. All of a sudden it clicked and my abilities exploded. I went from reading toad and frog to Lord of the Rings in a year. I read every book in my 3rd grade classroom and moved on to the school library. I pledged to myself that I'll buy her any book she asks for. My mom looked down on some of the sci-fi fantasy stuff I wanted as a kid, things that I was burning to read, I don't think it would have done me any harm. I've tried to keep some odd ball stuff she doesn't know she wants to read around, stuff that delighted me as a kid. Greek mythology, illustrated atlas's, Cool kids science books and what not. I don't think it's easy to guide a kids interest but dangling stuff in front of them and seeing what they bite at seems to be working.