I just bought this for my 9 year old and was so happy to see that had edited the previous version making a version for younger and older girls. It is excellent! First of all, it's branded American Girl which made my daughter love it without even opening the cover. It's like she knew she could trust everything it said.
I love how nicely it is organized and how it all leads up to the section on your body changing and getting your period. I thought that it simply described what is happening to her body and not to be afraid of it. I like how much time it devotes to the seemingly frivolous things such as pads, tampons, buying them, and how to be prepared. While yes, girls need to understand the biological facts what they really care about at this age is how to deal with it which this book does beautifully.
I wouldn't recommend giving this to your younger daughter without talking to her first, but it's a great guide and conversation starter - exactly why I purchased it.
I also loved how it devotes whole sections to overall health and putting the responsibility on her to get enough sleep, keep clean, get exercise, and make healthy food choices. Well done, AG!!