This book is mostly about different ways of thinking about models, in the context of making models of real-world phenomena. It dives into the cons and benefits of each mindset and tries to explain how knowing each is a good advantage.
This book is an easy and good introduction to recommendation systems. It is meant to introduce the concept of recommendation systems and, from there, give concepts and methods on how to make them. This book is about usage rather than the theoretical background.
Correlation: Any association between two random variables. Correlations are symmetric.