There are groups of people who are willing to buy nonfiction ebooks: hobbyists. At any given time, these people are looking for ways to spend their money on their hobbies. Their passion is your financial gain. What avid hobbyists want will always make great ebook material. Note that I did not say what hobbyists need. You may have certain opinions on what exactly certain people should need or should read. But those are not necessarily good topics ...
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