Puberty: "What's happening to me?" from Usbourne is quite good. Different versions for girls and boys, so make sure you get the right one. It goes into a reasonable amount of detail, so read it yourself first to help you decide what age you're happy for him to read it.
Sex ed: in my opinion, nothing measures up to the "Let's Talk About" series by Robie Harris. Three staged books aimed at different age ranges, very comprehensive and accurate information. We give our children the first book in the summer holidays before Y3, the second book the summer before Y5, and the third book the summer before secondary school.
Ah, sorry should have made it clearer. Yes I meant about puberty & sex ed. We were having the 'how babies are made' conversation & I had to tell him what sex was. He wasn't impressed! I figured its best to tell him when he's asking rather than wait till he is older & doesn't want to talk to me about these things. I also said to him, if anyone talks about sex and stuff in the playground & you don't understand, you can ask me. He replied 'why would people talk about that in the playground?' Lol. (as opposed to ds games & moshi monsters, I presume!)