I have two above average readers and this is so hard, because you want to challenge them, but yet, you don't want them reading about concepts that are too difficult for them. When my daughter was that age, she enjoyed Junie B. Jones, Animal Ark, Beverly Cleary and Boxcar Children. There is also a new Disney Fairy series out that she enjoys. A new series, Main St. in Camden Falls looks fairly good as well, I was told it was similar to the American Girl Series (which she also loves), but these might be too old for her concept wise yet. I believe there is currently only two books out in that series, but that might be something to build on year after year, just a thought.
A little OT, I was in charge of our Book Fair this past month and the book "Diary of a Wimpy Kid" FLEW off the shelf and I saw many kids who hate to read, reading that book!! So if anybody is reading this and has "non-readers", that might be a good choice to get them reading!!