Refactoring is no more than restructuring code by altering its internal structure without changing its external behavior. At first sight, it could seem that such innocuous transformations (from an external point of view) would not deserve a whole book. However, refactoring is one of the key techniques behind the success of agile software development.
Among other things, refactoring makes supple designs and test-driven development possible. When properly used, it helps in software maintenance (see Michael Feathers' "Working effectively with legacy code"). It also provides tangible benefits in debugging: if you don't find the error, that's because you don't understand what the code does, so making it clearer will always help.
Although you might find Fowler's catalog of refactorings rather dry and many integrated development environments automate them nowadays, thus making Fowler's recipes unnecessary, it is fair to recognize the milestone this book marked when it was first published a few years ago.
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