I recently released my new course: Find Your Freelance Writing Niches: Make More Money for Less Work.
It shows freelancers in detail why it’s important to specialize and how to identify the best areas for specialization. My main niche, of course, is writing supplements for textbooks, but I also have a secondary niche in outdoors and travel (two hiking guidebooks and over 100 newspaper and magazine articles.)
I answer many questions about choosing freelance writing niches in this post on my Productive Writers site, which also includes a link to purchase the course. (You can also click on the course cover in the sidebar of this site.)