How about Jan Karon's Mitford Series. They are clean? I also was thinking about the Redwall books, as another poster listed. Focus on the Family also has good clean books.

I am with you on having problems finding clean and appropriate advanced books... it is hard. I have a 7 year old who is reading fifth and sixth grade reading level books, and my background is in teaching, and I still have to hunt for books and be selective.

Good luck to you.