S.D.
I don't have a suggestion of a book but I have to say that I disagree with Kelly V's opinion of being too young and trying to keep him innocent. If you don't tell him and try to keep your son in the dark about issues like this he is going to likely learn from his peers and learn the wrong information at that. Let's face it-kids are starting earlier now. Yes, 8 is very early and it may not happen for awhile, but I can guarantee you it will happen and he will have the wrong picture if you don't explain it to him the right way first.
I would suggest asking him what questions he has on the subject that way you aren't going into anymore detail than he even has curiosity on. Chances are at 8 he won't have many questions other than the typical "where do babies come from" and "how do they get there".
Good Luck!!