Here are some of the questions a 9 1/2-year-old asks: "Mom, when did you go through puberty?" "Mom, when did you get your period?" "You mean you can get PREGNANT when you go through puberty????" "Mom, what's a tampon?" "Mom, what's anna...anna...anna...Anorexia?"
Here we go.
As a journalist, I have had mixed feelings about the American Girl line, mixed feelings I never had to confront as a mother because Daisy thought the dolls were creepy. However, they publish some fabulous books and one that is absolutely worth getting for your pre-pubescent daughter is The Care and Keeping of You. It covers all the above questions, plus basics like why you really, really do need to wash your face every morning.
I do wish they hadn't made the Asian girl on the cover quite so bowl-haired and slanty-eyed, though.
For those other birds and bees-type questions I've found the best books are It's Not the Stork (for boys and girls age 4+)
and It's So Amazing for children 7+
The Asians are better in these books, too.
I suggest having all of these on hand. You'll need them before you realize....