I read this book with my daughter when she was 11 (It’s great to be a girl…”) We read 1 chapter a week and we both thought it was good. Explained things clearly and appropriately.
The other book linked above, The Talk… Puberty, is good too. It is a bit more in depth, covers boys and girls, and Biblical in explanations. I have the series of books by the author and all are good.
Sandi Queen wrote a series of small paperback books. They are very conservative. The first one is From Girl …to Woman. It is $3.95, available at Rainbow Resource, as well as at the Queen Homeschool website. (They also have From Boy …to Man.)