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, starting with what information you need and then moving on to ways to use that information and interpret it to improve your results.
If you're ready to take your writing income up a notch, then this is the book for you.