I'm one of those who got your book via seeing you here or getting your newsletter, some sort of Medium-inspired contact. Your book is fine. I think it's the niche. I'm finding the niches that do well on Medium like "money" are not the same niches that make money for me on Amazon. Don't get me started on that Publisher Rocket guy. These guys who find keywords and niches don't help authors who know their subject. They help content mills locate keywords/niches so they can supersaturate those niches with junk. Some of them are teams who can fill the niche with multiple books within weeks, even days.