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.
M.L. Humphrey is a former stockbroker with a degree in Economics from Stanford and an MBA from Wharton who has spent close to twenty years as a regulator and consultant in the financial services industry.