This book is intended for Linux users who want to learn more about the inner workings of Linux and how the various pieces of the Operating System fit together. This book will guide you step-by-step in creating your own custom build Linux system from scratch, using nothing but the sources of software that are needed.
This book is also intended for Linux users who want to get away from the existing commercial and free distributions that are often too bloated. Using existing distributions also forces you to use the file system structure, boot script structure, etc. that they choose to use. With this book you can create your own structures and methods in exactly the way you like them (which can be based on the ones this book provides)
Also, if you have security concerns, you don't want to rely on pre-compiled packages. So instead, you want to compile all programs from scratch and install them yourself. That could be another reason why you would want to build a custom made Linux system.
Those are just a few out of many reasons why people want to build their own Linux system. If you're one of those people, this book is meant for you.