I think Mama laid an Egg by Babette Cole is a good non confronting one to open the conversation, especially with the younger ones. I too grew up with "Where did I come from?" by Peter Mayle. When my DD got a little older (8 or 9) I bought a book by Usborne that was pretty comprehensive but simple and cut down enough not to be scary! LOL They have a pink girl's and a blue boy's version of the book and it's called "What's happening to me?" (not related to the Peter Mayle book of the same title). Family Planning NSW has an online bookshop with a lot of resources in the "puberty" category.