I do not really see the point in Dave having to do more work for those who are too lazy to use the wealth of information already printed, unless Dave actually wanted to publish premium content for his own reasons.
I think Dave asked, because he wants to make some money from premium content. Nothing wrong with that. Only that in these times it won't take an hour until the premium content, if in electronic form, would be ripped-off and put on a torrent. And that would be it with making money.
If you read some past discussions, you will see that even Dave's forum has some people heavily advocating stealing / "pirating" content, and I don't think there will be any mercy with Dave's content.
So, premium content in electronic form would be a difficult thing if Dave wants to make money from it. Writing a book and distributing only paper copies has a slightly larger chance of surviving unpirated for some time. But eventually someone will run it through a scanner. And, as already discussed, writing a book is a time-consuming task, and even without it being pirated doesn't pay off.
Maybe Dave giving courses or doing some lectures for a fee, arranging fan meet and greats for a fee, anything that requires his presence would be a better way to deliver premium content.