Pick Your Pond: How Nonfiction Authors Can Find the Right Positioning | Jane Friedman

But this advice is tough to hear and act on. For me, the word niche summons an image of a small nook in a wall that might hold a single vase. It is, by definition, restrictive and confining. No one wants to crawl into a tiny box and commit to spending their career there. (That sounds boring!) You have grander ambitions for your book.

At its heart, "finding your niche" is about positioning yourself and your book in the eyes of others. You do need to differentiate your book from the thousands of others on your topic. If you're building a career around your book, be able to explain your specific take on the subject. If you're building a broader author career, over time you will differentiate yourself as well.