Best book about puberty/sex?

[JustCallMeBaldrick]

I'm after recommendations for a book for DS (9), as although he knows the very basics -egg and seed meet - he hasn't asked about the mechanics of it all, and I don't want it to be a huge surprise/shock when he covers it in school!
We talk openly at home about things, but as he hasn't asked I'm not sure how to start the conversation...

When I was younger we used 'the body' book and I used this along with a more detailed book when I was pregnant to help answer the question that my niece (5) and nephew (8) was asking xx