It's not a thick book, but it covers everything. My ds would rather read a book and ask me questions he has rather than an in depth talk from me or his dad. We've always tried to answer any questions he's asked over the years, clarification of things he may have heard on the playground etc, but I found this book quite good at covering most other things he may need to know or is curious about.
My dd has a copy of the girls version of the book recommend above. It's pretty good (and I've taught quite a lot of sex Ed so got a good idea of what tweens ask about this topic). My dd dips into it and caught her the other day reading it at bedtime. We have sat and read parts of it together as well so totally recommend it too. She's 10 at the moment.