Refactoring: Improving the Design of Existing Code - Martin Fowler

Overall

A fairly old book, it was using Java 1.2! This makes it somewhat outdated as many code samples are awkward with the latest Java features. Despite the age, authors managed to convey some solid experience of refactoring to the readers, if not theories.

Where it shines

The book did a good job justifying the reason for refactoring and its relationship with testing. Book highlights refactoring as the first step before introducing any additional feature to production software, argued that this will only make the new feature done faster.

One more opinion is that refactoring should not be mixed up with adding new features.