Refactoring Improving the Design of Existing Code second edition by Martin Fowler

"Whenever you read [Refactoring], it’s time to read it again. And if you haven’t read it yet, 
please do before writing another line of code." –David Heinemeier Hansson, Creator of Ruby on Rails, Founder & CTO at Basecamp

Fully Revised and Updated—Includes New Refactoring's and Code Examples

“Any fool can write code that a computer can understand. Good programmers write code that humans can understand.”
–M. Fowler (1999)

For more than twenty years, experienced programmers worldwide have relied on 
Martin Fowler’s Refactoring to improve the design of existing code and to enhance software maintainability, 
as well as to make existing code easier to understand.

This eagerly awaited new edition has been fully updated to reflect crucial changes in 
the programming landscape. Refactoring, Second Edition, features an updated catalog of 
refactoring's and includes JavaScript code examples, as well as new functional examples that demonstrate refactoring without classes.

Like the original, this edition explains what refactoring is; why you should refactor; 
how to recognize code that needs refactoring; and how to actually do it successfully, no matter what language you use.