I don't know of any good books, but I did talk about sex with my two oldest ones at age 9. I have 4 kids, they are 13,11,7 and 1. The school district they attend has a program in the 4th grade where they show a video, not about sex, but explaining about their bodies and the changes. They usually have to ask for parents permission in order for the kids to watch the video. I gave permission for them to watch the video and then I made sure that on that same day, I asked them questions and discussed what they saw on the video and asked them if they had any questions. It was nothing graphic, it makes it easier to break the ice. Of course they will ask more questions as they get older, but the key is to keep that communication open between you and your kids. Our school district is great about directing the kids to practice abstenance. Check with your school district, sometimes they have parent worshops and meetings on this kind of information.
R. M.