The Ten-Copy Rule
A simple litmus test on whether your book's focus was sufficiently specific.

The author who writes a book "for everyone" really just penned a bunch of pages for no one. If you only remember a few words from this post, they should be, "Maintain your focus throughout the writing process."
Seems obvious, right? Yes, but many writers wind up penning entire chapters that, while interesting on some level, don't matter to the book's intended audience. For this reason, it behooves authors to develop detailed reader personas long before even outlining the book, much less writing its first draft. If you aren't sure about how to do this, find a book coach who will keep you on track.