Look for weaknesses in current offerings. What's something you WISH was around when you were learning about software development? Or something you hear budding developers complain about?
As far as book vs. course: Personally, I would work on a course and then use those materials to make a book. But that's just me. You could make a short and free "fundamentals" course to test the waters.
As far as book vs. course: Personally, I would work on a course and then use those materials to make a book. But that's just me. You could make a short and free "fundamentals" course to test the waters.