So, we went to K - 4th grade books from years past for my voracious reader when she was in K. The content felt a little safer because I didn't want to pre-screen the books.
Nancy Drew, Stuart Little, Charlotte's Web, The Moffats, Little House on the Prairie, Homer Price, Sarah Plain and Tall.
There is also the Magic Tree House series, not deep reading but gives a little history. And, the American Girl books were a hit.
We also continued with picture books, and I think they are still okay for advanced readers. I was always an advanced reader who would read anything as a kid. I read college level books in 4th grade. But, that DID NOT mean that I didn't enjoy pictures books in 4th grade. I still found them fun. I wouldn't have wanted to read them all the time, but every now and then I found a good story. So, even though she is way ahead there are still some good picture books out there. "Sylvester and the Magic Pebble" comes to mind. I also use to love a collection of Fairy Tales. It was a big book, but had lots great stories with a moral. And, some of the older books throw in the occasional throw-back words that up the reading level.
Edited to Add: One of my kids, who was obsessed with science, read middle-school science books in K. I think if you find an interesting science or history topic, you can also up the reading level there.
Yep, many of these books have already been through the rotation. Magic Treehouse was a big favorite when she was 4 and 5, but DD has started to lose interest in them over the last 6 months. I completely forgot about Homer Price, that might be a great fit, thanks!
Charlotte's Web is her favorite book, for a year running. She has the whole thing memorized!!
I will have to look up Sylvester and the Magic Pebble, that looks great. DD still enjoys picture books, and since so many are written to be read by adults, it is easier to find books with advanced vocabulary. She does read a lot of fairy tales.