I just found out about a very interesting "where did I come from" type book called "What Makes a Baby?"The unique thing about this particular book is that it is not about "a mommy and a daddy who love each other and make a baby", as is typical to find in these type of books for little ones to explain about biology and family making. This book makes room for all the possible types of ways a baby may come about in the world now a days.
http://www.what-makes-a-baby.com/http://www.what-makes-a-baby.com/buy-the-book/