The world of books is saturated.
Amazon now has something like 12 million books available. But there are only about 12 million really dedicated, addicted readers in this country. If each of them read 100 books a year, and if all books sold the same, that would mean your masterpiece would sell 100 copies a year. That would provide you with one or two hundred bucks a year in income. Not enough to buy a Starbucks frap more than once a week. Double the number of readers, the picture doesn't change much.
What's worse is that the above scenario is very generous. The reason is that most of the books in readers' Kindles or on their bookshelves are written by famous authors. So unknown authors have a hard time getting anyone to look at their book. In fact, I looked at one of the popular internet publishers. (Note that I strongly recommend you avoid using internet publishers.) I divided the total books reported sold by the total number of titles available. The result? The average novel they publish has sold less than two copies.
By contrast, I once read in Publishers Weekly that the average novel published by print publishers sells 80 copies. Big by comparison, but still very bleak.
Where are the authors whose books sell enough to earn a living? They're out there, and we know their names. But they are a tiny percentage of all authors.
Does this mean that all those books by unknown authors are no good? Not at all. Some are masterpieces. This is simply the reality of too many books chasing too few readers. Which brings us to the new number one rule of writing success.
Find a narrow niche and own it.
This is the big fish/little pond concept. Let's say you've written a romance or mystery or fantasy. Or maybe you've invented a new genre. Further, let's say your book is really good and has a fascinating, sympathetic character in major trouble going up against one of the all-time greatest villains. David vs. Goliath. Write a great book and readers will come, right?
Unfortunately, millions of other authors feel the same way about their book. Every year there are another million titles out there. Another romance or mystery or fantasy or mainstream novel has almost no chance of making a blip on the graph.
Which authors have the greatest chance of finding an audience? Those who choose a niche that is narrow enough that their book pops up whenever someone looks for something in that niche.
Let's look at examples. Let's say someone wants to read a great thriller with a racing plot and heroic characters and a world's-about-to-end story line. What would they do? Search for that description on Amazon? Or Google? Probably not. More likely, the reader would think of a subject they really enjoy and search for that. Religious mysteries. Or time-travel fantasy. Or archaeology thrillers. Or twisted psychological novels. If so, would your book pop up? No. More likely they would find a book by Dan Brown or Diana Gabaldon or James Rollins or Gillian Flynn.
The problem is that general search terms only bring up books that are already popular. Most authors who try to write those kinds of books never find any audience. There is too much competition and the field is utterly dominated by current big-name authors.
If, however, you write a book series that has an unusual story thread about, let's say, a history book club of elderly women who find a way to time-travel to the time just before Christ and, using their knowledge of how historical movements survive, help Caesar come to power. If you write that, you stand a very good chance of owning that niche by the time you've published just a few books in the series.
Is the audience for this subject large? Probably not. At least, not yet. But there are people who love to read about the Roman Empire, and there are readers who love time travel. Those readers will periodically search on such subjects. Whenever their combined search terms get close to your books, they will pop up, often at the top of the search results. The reason is simply that your books will be the only series with this unique niche.
Assuming your books are well written and professionally edited and have professional covers, those few early readers will spread the word. Eventually, your narrow niche will grow. It may even become popular enough that other writers will try to adopt your niche, which simply gives it more credibility. Whatever attention those writers are able to get will build your audience. Everyone will compare their books to yours because you created the niche. In any discussion of those other writers, you will get mentioned. You will own your niche.
You may be wondering, "But what if no one cares about my narrow subject and my book doesn't take off?" That may be the case. But if you try to be one more of the 12 million minnows in the big pond, that will almost certainly be the case. Choosing a narrow niche at least gives you a chance.
This approach doesn't just work with writing books, either. Jeff Bezos could have decided to jump into the big pond and open a regular bookstore just like everyone else. Instead, he chose a niche so narrow that no one had ever thought of it. Selling books online. What kind of nut case was he? But he immediately became the biggest fish in what was then a very small pond. Of course, that pond grew a bit. And every imitator that came along only boosted his rep.
So how do you pick a niche for your book series? The obvious ones are easy.
Pick a setting that is not featured in any other series. (Just Google your idea to find out - "books set in Anytown, Arkansas." ) Every person who grew up in, or is otherwise interested in, Anytown will be intrigued by your books. And people who've never heard of Anytown will be curious about books set in such an obscure place.
Pick an occupation for your protagonist that isn't featured in any other books. Yes, the obvious occupations, like Medical Examiner, are already over-represented. But has anyone written a series featuring an ex-convict woman who makes hand-built, wooden sailing yachts that are prized by smugglers? You get the idea.
Pick a time-and-subject combination that isn't featured in any other books. A gay circus strongman set in the 16th century. A woman physicist who was Einstein's secret paramour and who also gave him many of his best ideas. A mother who's never slept with any man but her husband but discovers that the DNA of her children came from another man. An astronomer who discovers an intelligent laser transmission from outside of the solar system, a transmission that comes from himself in the future. (Maybe that one's been done - I'm not a SciFi expert.) The point is that it's relatively easy to come up with unusual ideas that can be intriguing.
What if you've already written the first novel or two in a series? It's not hard to retrofit them into a narrow niche. Look at your story arc and your characters and consider how they might be changed to take your novel from one of the masses to one that's unique. You might be surprised at how easy it is. The reason is that all stories have similar basic components. Changing the costumes and the stage and some of the themes is not that difficult. Bringing your novel into a narrow niche will distinguish it from a million others.
If you find a narrow niche and own it, you will find an audience. Then the only question is how big you can grow it.