In addition to all of the above great advice, I would try to get them out sailing in dinghies and smaller boats. I think the effects of sail trim and adjustments are more apparent and much easier to see in a small boat where you get immediate physical feedback from the boat compared to a bigger cat or mono. And they are a lot of fun too.

As far as books, I was pretty impressed with Nauticed when I used it last year for an online chart briefing. I also find myself referring to my old ASA material whenever I need a quick refresher for myself or have newer sailors on board. However, nothing can replace a series of hands on lessons with a certified instructor when it comes to learning how to sail and be safe.