I like "What's Happening to My Body? Book For Boys" too. I've seen others that don't discuss intact penises, or all the drawings represect circ'd penises, so I was really happy to see that this book discusses the function of the foreskin and how a intact penis changes during puberty. The book contains info on puberty in girls, sex, birth control, acne, masturbation, growth spurts etc. There is a ton of info in the book, at 10 your son may not "need" all the info yet, but it's not inappropriate either IMO. A lot of the reviews on amazon say that it's more appropriate for older teens, but I think boys need this info way earlier than that. We've always been really open with our boys about sex, maybe that's why I don't think that book is "too informative" for younger boys. My newly 9yo has looked through the book. I directed him to a bit of info (he's starting to get stinky pits, and had some questions about sperm) He read a bit of the book, but wasn't interested yet in all the info.
We have it laying around so he can look at it when he has Q's.