From where I'm sitting right now, I can see two books resting my coffee table. One is Ta-Nehisi Coates's book Between the World and Me, his experience of race in America told through the vehicle of a long-form letter to his son. The other is an introduction to spiritual formation by a man who spent his adult life teaching on the subject at a seminary.
As I look at these books, I know only those two men could have written those two books.
No one else could have framed the black American experience in exactly the way Ta-Nehisi did, given his own experience, and no one else could have written the same exact book to his son, one spoken with a voice that combines all the intimacy, love, fear, and honesty only he and his son know together.
And no one else could have written that same introduction to spiritual formation as this other man did — someone who lived it for decades and taught it for decades, too, and knew how to set it down in a voice that is accessible to the rest of us and gives us the small and big picture, together.
Many have written and will write books on those same two subjects. But only those two men could have written those exact two books, given their experience and intent.
You have authority like that too. You have experience and knowledge and a voice that can't be replicated.
Write the book only you can write, writer. Say what only you can say.