The New Parenting Dilema~Need Book Reccomendations

As many of you know my daughter is 7 years old and in the second grade.  She LOVES to read.  Here the schools have the kids do an AR program and keep track of their reading level.  So they hand out books to the kids and the children take a test on the book they read and it is tracked.  It is a good program and it gets the kids reading and helps the ones who aren't reading as well as they could be.
However, I am not alone in this dilema it is happening with preteens I know as well.
My daughter is reading on the 5th grade 6th month level.  She flies through books.  We have been through Junie B Jones, Amber Brown, American Girl, A to Z Mysteries, Hank the Cowdog, My Life As A ___, Magic Treehouse and MANY more.  There are series that are geared towards girls at the reading level she is on, but the are not targeted to her age and deal with things like puberty and boys.  The girls my daughter's age don't need to be reading about puberty issues, and I fear it will make her grow up faster than she should.
We tried The Boxcar Children and she couldn't get into it.  The more scary the book is the less likely she will read it. 
Recently I bought her The 39 Ckues Book One The Maze of Bones and it is a new series that you read the books, play the game online, and can win prizes.  I personally read the book cover to cover in one sitting which is odd for me.  I love the book and I am the one who can't wait for the second one to come out.  My best explanation of it is it is National Treasure like blended with some Harry Potter themes.  She won't even look at a page in the book, because the cover laden with bones scares her.
We parents are scratching our heads and trying all kinds of books like Nancy Drew, The Babysitters Club and others we recall from our childhood, but they are harder for the younger kids to relate to as so much has changed with the times.
What in the world do we hand these girls to read that is challenging but appropriate to their age and relevant to them???