I wouldn't even consider this book, a book. I think of Working Effectively with Legacy Code as a tool.
Well, a tool is something you use to accomplish a specific task. This is exactly the way I use this book. Because you can write legacy yourself, it's very important to refactor from time to time. This is when you take your refactoring shovel out of the shed and start digging. Well, think of this book as an extension to your refactoring shovel.
Reach out for this book whenever your boundaries seem fuzzy or when you have a piece of software that seems right, works almost right but doesn't feel quite right.