My mom bought me this book when I was 11 or 12 because she knew me well enough to know I'd be awkward as hell to talk about it but if it was in a book I'd devour the information.
It was a great book. Probably super dated now (this was when the Internet was young), but still pretty comprehensive. I knew way more about sex than all my friends, even though they pretty much all had it before I did.
I had this book growing up and it was amazingly helpful!! It is a little dated ( 1999) but I still think it would be very helpful. It covers a wide range of topics related to being a young woman.
"Deal with It! A Whole New Approach to Your Body, Brain, and Life as a gURL"
http://www.amazon.com/Deal-Whole-Approach-Your-Brain/dp/0671041576
I bought mine myself when I was 15-16 online with a visa giftcard. There are enough ways to bypass you and for her to get it herself (assuming she has some pocket money) if you don't.
As for what would be age appropriate, I'd be careful about how good of a vibe you get her. Lots of grown women overdo it with vibes and have trouble climaxing with a partner, and it'd be easy to do that with all the teenaged hormones floating around.
I had this book when I was in my early teens and found it immensely informative and helpful It's not all about sex but the chapters on sex and relationships were of particular interest to me.