For book/ebook authors, publishers, & self-publishers
The most effective tool I've found for researching a niche is a $37 software program called Search Automator.
I own a privately branded version of the program, and you can search on a term (niche) and will get results on websites, videos, resources, blogs, newsletters, ebooks, guides, reviews, forums, training, courses, events, software, etc. Basically it's everything you'd need to know to research a niche. This is an excellent tool if you're trying to determine or refocus your target market, and is especially helpful if you want to create a membership website.
Research your niche here.