The source code is often the only accurate description of the software. On many projects, the only documentation available to programmers is the code itself. Requirements specifications and design documents can go out of date, but the source code is always up to date. Consequently, it's imperative that the code be of the highest possible quality.

Code Complete: A Practical Handbook of Software Construction by Steve C McConnell

ISBN: 1556154844 This book is available from Amazon