My daughter is 9, and will be 10 this year. 5th Grade.
I got her, the American Girl book series. Its good. She likes it.
I also talk to her about it. Don't just let your girl read this book... it is also meant to be talked about WITH mom.
It is a very good book.
AND in school, per this grade level/ per my daughter's school, they will have a "class" (boys & girls separately), about how their body will/is changing etc. With parental permission forms sent home etc.
Some 9 year old girls, get their periods already.
They have body odor.
Some have boobs already.
For boys, I don't know specific books. But just do a search on Amazon... because then you can also read the reviews of the books.
Oh and, also... make sure to explain to a daughter, about how all these bodily changes, also means hormone changes, AND it will affect their moods. Boys too. My friend said her son gets the male version of moodiness- ie: he gets surly. At certain times.