Should I write more books as a grad student, or focus more on studies/research? I started a blog that attracted the attention of publishers and they began offering me contracts to create video courses for them. I signed on with the first legitimate publisher to contact me last year and I've already published three courses with them (I owe them one more). I got paid advances and hopefully at some point I'll start seeing royalty checks too (I don't get paid until royalties exceed the advance; fair enough).

Writing and recording videos for these courses though is a lot of work and the publisher hounds me about deadlines. I thought after I publish these courses that would be the end of it, but other (and in my opinion bigger and better) publishers have taken notice too and they are wanting to get deals too.

I'm starting to stress out about whether I should try to squeeze a couple more books/courses in, maybe try to hurry through them during the summer, or if I should just say no more until I complete my Ph.D. (or at least my thesis and I'm basically coasting to graduation). The work is stressful and a distraction from studies and research, but the money I'm paid as a grad student gets me a small apartment and no car and money is still tight, and I would love to have a little more income from royalties that could maybe make a car payment or even an apartment in a better location (one where there's actually fun places to go in walking distance; I live alone and I'm getting lonely, and my location is a big part of the problem IMO; it feels impossible to meet new people here when a grocery store is the most interesting place in walking distance). I also have this fear that if I tell publishers no, I'm "missing out" on something that will pay off in the future.

TL;DR: Should I try to squeeze out a couple more video courses/books with publishers, or say "no more" until I finish my PhD and have a good research program going for me?
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]".