Refactoring

Every software module has three functions. First, there is the function it performs while executing. This function is the reason for the module's existence. The second function of a module is to afford change. Almost all modules will change in the course of their lives, and it is the responsibility of the developers to make sure that such changes are as simple as possible to make. A module that is hard to change ir broken and needs fixing, even though it works. The third function of a module is to communicate to its readers. Developers unfamiliar with the module should be able to read and understand it without undue mental gymnastics. A module that does not communicate is broken and needs to be fixed.

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I can't stress this enough. All the principles and patterns in this book come to naught if the code they are employed within is a mess. Before investing in principles and patterns, invest in clean code.

Robert C. Martin, "Refactoring", in Agile Software Development:
Principles, Patterns, and Practices, 31-42.