It is written with a lot of thought and care. My twins are just 4, so we don't need it with urgency, but should the need arise in the near future, we could read this one to answer some of their questions. The book was first written in 1999, so some bits might feel/sound a little dated. Feel free to tweak them. But in all honesty, if you are only starting to prepare to have this conversation with your child, this book will work as a good beginner template.
It's a good starting point to introducing 'the facts of life' and helping you to explain to a young child "where babies come from". It's clear that a lot of thought went into developing this book as there are good size comparisons made that a young child will understand; and care has been taken in describing labour contractions and cesarean sections so as not to worry a child.