If you want to make money from your writing then you need to treat it like a business. And part of treating it like a business is looking at data about how that business is performing and using it to inform your business choices.
In Data Analysis for Self-Publishers M.L. Humphrey walks you through how to do that. What information you need, ways to use that information, and what the results of your analysis may mean in terms of what to do next.
If you're ready to take your writing income up a notch and haven't been doing any of this before, then this is the book for you.