I used to run workshops on gender issues and girl empowerment for 5th graders at a summer camp.
Probably the best book I used with girls in our empowerment workshops was called Baby X: A Fabulous Child’s Story. I think it is out of print but I looked online and you can read and print the whole short story here: http://www.gendercentre.org.au/22article4.htm
Definitely a good one for sparking discussion…
I would second the suggestion of Weetzie Bat – really awesome feministy and hip and fun and mysterious and special – she might be a little young to appreciate it – you should read it first and see what you think. You can read it in one day, and it’s a great book for adults too.
Also, you might want to buy her a book like Our Bodies, Our Selves. She may think it’s weird to receive it now if she’s only in 5th grade – but I guarantee she will pour over it when alone in her room, hoping to find answers to all the questions she is too embarassed to ask. Hopefully it will counteract the BS her friends will find on the internet.
And finally, I know you said you’re not looking for fiction but you might want to check out The Paper Bag Princess. It’s a picture book, but I think adults can appreciate it too.