This is a book about how to detect bad code from code base with the help of code's history from git. I was surprised how simple things and techniques book uncovered about code quality. The key point is the time. We shouldn't do just static analysis from our current code but analyze how our code has lived and progressed. Tornhill calls this behavioral code analysis. I will explain some interesting things from the book, like hotspot analysis and what are surprises in code and why they can be really expensive. After reading the book I understood how to improve code quality cost-effectively.