Pro Book that focuses existing codebases
Many books focus on new greenfield code base. How to create new code with best practices in mind. This book on contrary tries to help developers to deal with the code that is old, hard to change without the fear of breaking and not covered by any tests.
Con Does not offer "the best" solution
The techniques from this code might not be the cleanest solution that you might get from the others books, but it doesn't aim to provide one. These can be hacks, code smells and just nasty code, but they are better that what was there before and allow you to take your code to a testable state. After you get your code covered by tests, then you can refactor and re-architect it to something better. This book provides a place to start in situations where many may think only a full-rewrite is an option.