L.S.
I don't really have any books to recommend. But I wanted to chime in to say that I don't think that 9 is too young at all. I bet he knows more than you think he does! When my son was 9 I was his cub scout den leader, and we took a field trip to some of the Philly sports arenas which included a tour of the 76ers locker room. At the time, along the walls there were huge pictures of the sixers cheerleaders, and the comments I heard from some of these boys were unbelievable! Over the course of their third grade year, the boys made all sorts of sexual remarks until finally I had a meeting with the parents to tell them exactly how their boys were behaving.
I'm not saying that's the case with your son or his friends. But you only need one precocious child in school to start talking about all this stuff. What's most distressing is that they, of course, being immature, spread information without any of the respect that human sexuality deserves. That's just how they deal with this amazing information.
So however you talk to your son, do try to convey a respectful attitude about sex. And don't make it a one-time discussion, either. Always be open to conversations he may want to have, and bring things up yourself from time to time. I'm still talking to my teenagers about sex, but now our conversations are more about the emotional side of it, and every day and in every way my husband try to show the boys how men and women should treat each other respectfully. I think it must be working, because I was talking to the mother of my 18 year-old's girlfriend the other day, and she had to tell me how much she loves my son because of the respect he shows to her daughter. That made me feel wonderful! (And by the way, I think she did a wonderful job with her daughter, as well.)