Should I write more books as a grad student, or focus more on studies/research?
Only you can answer this question. What exactly do you want to do after you graduate? Which is more important to achieving that: the PhD, or the money? 

I'll only point out three things you might not have considered: first, if you're already in this much demand, you could conceivably quit the PhD and make creating video courses your full-time job. Second, you could tell the publishers you need to finish your PhD, and that you'll be willing to do this afterwards. Finally, when it comes to education, the credentials of the teacher is a big draw. Yes you might not be teaching something hard enough to require a PhD, but there's still something to be said about "the teacher cgmil earned a PhD in [the topic being taught] from [some university especially if it's a top school]".