The secret (It’s not really a secret – you can find this all over the internet) to making money off of this in the book world is to have lots of books – preferably in the same series. Then, some portion (but not all) of the customers who pick up one book will buy all of your books, or at least all of the series.This is echoed by Nick Cole, author of Ctrl-Alt-Revolt! and The Wyrd Chronicles:
Write a three book series and don't release until Book 3 is done. Rinse and repeat.These are both working authors, so you should definitely listen to them if you want to be a working author. Do as they say, not as I do. I'm working on my fifth book and my fourth novel length story, and every single one of them is different from the other. I just can't help it. Once I get a story out in one universe, I'm already looking around for the next universe to explore.
The dirty little secret is that I don't world-build. I story-build. The background of every one of my books contains just enough information to paint a broad picture of the situation, and enough background to drive the plot or motivate the characters, but for the most part my settings are designed to fit the story and not the other way around. I've actually gone back and changed entire universes in a few instances in order to bend them to the needs of the story.
It might make good financial sense, but I prefer the wide-open spaces of tight and compact story telling over sprawling epics.