Hi bunz, and welcome.
I used Let's Talk About Sex by Robie Harris for my son with AS - he's now 13 and we started using the book at around 8 years, revisiting it as and when needed.
But as ever with children on the autistic spectrum, it's never as straightforward as just going through a book. I found I needed to be alert to any changes in temperament, anxiety etc and know when to end the discussion or explain in more/less depth. I also supplemented with tv programmes where appropriate.
I guess I dealt with the general subject with openness and fact, which was what he could cope with best at the time, and introduced the emotional aspects later when he was ready for it. Other children might prefer to do it the other way around but ds was/is very science-minded so the primarily factual approach worked best for him. He's now just as interested in how people tick so we look at it from that angle too.
Overall, I found that as I'd started off being open and having no embarrassment or discomfort around the subject, he's much the same so will ask me anything he wants to know.
That said, with an AS child there's a lot more to it than simply 'sex education' - you can't really separate it from other areas of personal development - so you'd need to consider incorporating elements like personal space, appropriate interaction, personal preference and difference etc as it's all linked IMO.