Hi. I’ve been using Scrivener for Mac for a few years now, but in terms of computer skills I think I still qualify as a Dummy. 
Since your book is supposed to be for Dummies, please don’t forget to include the real basics. Things you might think any idiot can figure out on their own from looking at the screen, so why should you include it… but you should.
I’m talking about things like how to change the font and font size, individually and as default settings; the fact that the word count of whatever section you’re in is shown at the bottom center (took me a long time to notice this), or that even when you’re in full-screen mode, if you run your cursor up to the top of the screen the menu bar will appear (I used full-screen for over a year before I found this out, and I didn’t discover it myself - Keith told me in this forum); how to customize the toolbar, etc.
Another thing that confused the hell out of me for a while was when one day my project word count mysteriously got about 30,000 words shorter. I finally figured out that the target thing only counts the bits that are included in the compile (at the least the way it was set at that time), and I had changed what bits I was compiling.
Someone else also mentioned this, but basic keyboard shortcuts would be really great. Even if they are not unique to Scrivener (i.e., if they are basic Mac shortcuts), but are things that people use often in Scrivener. For example, when I’m working in full-screen mode, I like to pull up the font color box and the target box and have them off to the side at all times. So I need cmd-shift-T and cmd-shift-C for those.
I can’t think of anything else yet, but please do include really basic things and be sure to explain them so that true Dummies can understand. Thanks and good luck with your book! 