The goal in this book is to find the one metric that has led to or will lead to the growth of your company. The one metric that matters (OMTM).
Good metrics are comparative, understandable, a ratio or a rate, and change the way you behave.
Examples of a good metric from Airbnb is the quality of their photos on their site, which led (or at least correlated) to exponential growth in 2011.
Figure out what type of e-commerce business you are in acquisition mode (repurchase rates under 15%), hybrid mode (repurchase rates 15-30%), or loyalty mode (over 30%) like Amazon.