Hi G.,
The American Girl's book "The Care and Keeping of You" is excellent! Lots of bright pictures, short information sections, easy to read text for kids and adults. We are using it with our 9 1/2 yr. old who is starting to go through puberty, and her 8 yr. old sister since she probably won't be far behind, and it can't hurt her to start learning about it.
We gave the book to them about a year ago and read through some of the body basics with them (hair care, bathing, healthy eating, exercising, etc.), to show them that it has lots of good information that they can find out on their own. We also let them read it on their own and then have open discussions about what they have read. It has worked great for us. I think the puberty/menstruation talk is a little uncomfortable no matter what, but to have a book to refer to has been very helpful.
The girl's dad and I also like this book because it does not go into all the sex stuff. So we can cover that at a different time, or as it comes up in discussions through this book, which some things just naturally have. But it lets us pace the sex talks the way we would like to and need to depending on what the girls are really wondering about, rather than because they saw or read something in a book that they may not be ready for.
They do seem young, but I think it is important to have "mini" discussions whenever the opportunity comes up. That helps it all to just be a part of life, rather than the BIG DEAL that it seems to be for so many of us. I think it also helps the kids and the parents to be more relaxed about the topic when it is spoken about as a natural part of life rather than waiting for one day to have "The Talk." The more often it's done, the easier it gets! :)
Sorry this is so long, hopefully it is helpful.