At nearly 10 I think it is perfectly appropriate for him to be reading about adolescence and so on. A lot of girls and some boys in his class will be hitting puberty in the next year or two, and schools usually do basic sex-ed (mainly 'growing up' related, but also stuff about sex) in year six, though the puberty bit is too late for some of them by then.
If it is one of the Usborne books about the human body, or particularly 'What's happening to me?' they are ideal for that age. I bought DS the boy's version when he was about ten, and DD had the girl's one at six or seven.
I wouldn't worry. The stuff he doesn't get he will just edit out and the stuff he may think is interesting will be worth a discussion.
I remember accidentally getting a 1960's gay porn novel from a car boot sale at much the same age and just not getting it. It "disappeared" from my bookshelf a few months later. However i just liked reading so it was the words and sentences I was interested in rather than content if that makes sense.
Mine is the same age and we will have to do the whole "wet dreams" conversation soon I think so it's not horrendous when it happens in a few years.
My son is 8, nearly 9. I think he knows quite a lot about sex already actually- all learned from the playground, alrhough I have always referred to where babies come from, and how they are made (but not in any detail).
So I reckon your son may already know some of the mechanics- it us the emotional stuff that is more difficult, and which is ultimately most important for us to share with them. But is 10 to early for that? Probably not- I always say to my son it is important that you live and care about anyone you have sex with. Not sure if he takes it on board, but I reckon the repetition must go in somewhere eventually!
Think its the usborne one mentioned above, its called 'whats happening to me' my ds has had it to read since age 8 ( I bought it for him when he told me he'd been to his dads house and his step sisters age 16/17 wore 'nappies' and babies came from the gooseberry bush according to his step mum!!!!) Its really clear and easy to read/share without embarrassment.
The Osbourne book of growing up is really good for that age group. My DS has had it since then. It covers puberty, and the basics of sex, but not in great detail. If the book you've got is mainly science-focussed, then that's probably fine if he's interested in how the body works, then no reason not to understand reproduction. The relationships content might be a bit unsuitable though.
In terms of suitable reading, he class teacher should be able to point him towards things that are right for him both in terms of content and suitability. Or as a librarian.
My DS has a reading age really advanced for his age I was told to get him nonfiction book . Because the story's that are at his reading level 15 years + will have content that at 9 he maybe should not read lol . So I got him a set of nonfiction books about science . 1st chapter ADOLESCENCE ,REPRODUCTION AND RELATIONSHIPS . grate !!!! I just am not sure when do boys start to think about that stuff and what at almost 10 should I be telling him !