A few families have asked for suggestions to grade-level appropriate reading content. Here are a few links to book lists. I have not read all of the books listed, but wanted to make the lists available to you to look over. I suggest looking into nonfiction in addition to fiction.
Great Schools List
Charlotte Mecklenburg Library - Nonfiction
Darien Library - Nonfiction
Good Reads
California Reading List Search - Select a grade range and a reading list. The higher the reading list number the more challenging the books.
Common Core Toolbox - Recommendations here are intended for guided reading, an adult reading with the child, but may also be beneficial for advanced independent readers
Scholastic Common Core Search