This really depends on your child, and the relationship you have with your child. Parents need to discuss many things more openly with their children from a younger age, simply to build trust and keep open lines of communication. From my experience this is not a one time conversation, and rather a series of conversations (that I am still going through). It can start with changes to our body when they need to start wearing deoderant or when a girl starts shaving her legs. Explain how their bodies change, and why they change. Sometimes books can be good, but it should not supplement a conversation. You can use it to start a conversation, "I got this book about some things you might experience, after you have a chance to look at it, if you have any questions lets talk!"