"The Old Man and the Sea could have been over a thousand pages long and had every character in the village in it and all the processes of how they made their living, were born, educated, bore children, et cetera," Ernest Hemingway told The Paris Review. "That is done excellently and well by other writers. In writing you are limited by what has already been done satisfactorily. So I have tried to learn to do something else."
This is the most common concern among others: Has this book been done before? Nobody wants to spend a year or two writing something already written. Don't worry! Only the most cursory exploration of a topic would ever align that closely with someone else's. We're all just too peculiar. But you must plumb the depths. Read and learn and think about something for long enough and you can't help but write a unique book. It just works out that way.
"I always try to write on the principle of the iceberg," Hemingway said. "There is seven-eighths of it underwater for every part that shows. Anything you know you can eliminate and it only strengthens your iceberg." [Emphasis mine.] Hemingway conducted an enormous amount of research before writing his novels. Most of those details never found their way into the manuscript. Hemingway understood that what he understood mattered as much as what he put on the page. Think of the great works of literature, music, film, or art. What isn't included is just as important as what is. As Miles Davis put it, "It's not the notes you play, it's the notes you don't play."
Crucially, you have to know what you're leaving out. Your audience can tell the difference between an absence and a gap: "If a writer omits something because he does not know it then there is a hole in the story." Hemingway's novels convey so much and feel so rich with so few words because of everything Hemingway knew and chose to leave out: "I’ve seen the marlin mate and know about that so I leave that out. I’ve seen a school (or pod) of more than fifty sperm whales in that same stretch of water and once harpooned one nearly sixty feet in length and lost him. So I left that out. All the stories I know from the fishing village I leave out. But the knowledge is what makes the underwater part of the iceberg."
This explains the sophomore slumps of best-selling authors. The first book represents the culmination of years of thought, research, and practice. The second, rushed out to capitalize on the success of the first, may be longer, more complex, and more ambitious, but it's perched on a smaller iceberg. As a result, it's unstable. Shaky.
As Hemingway advised, "Strengthen your iceberg." Most of what you discover in your exploration won't make it into your book. Maybe even seven-eighths of it, as Hemingway suggests. If you've gathered enough material to write eight good books, you may be ready to write a great one.