Short answer: a book's real micro-niche is almost never the broad genre or topic you'd put on a jacket flap — it's the specific intersection between your topic and the exact problem it solves for a defined group of people. That intersection is found by observing communities, not guessing from your desk.
Don't search for book mentions — search for complaints, repeated questions, and phrases like "I can't find anything that explains X for Y" or "every book on this topic assumes you already know Z." Those phrases are literally the market gap your book can fill.
Once you spot that specific gap, that's your primary long-tail keyword, your book description's angle, and probably the title of your first related blog article.