I recently inherited a very large code base, which was mostly a hodgepodge of spaghetti code, full of tightly-coupled untestable components. Luckily, around the same time I discovered Robotlegs. To say that this framework saved my sanity is an understatement. The framework is great for new projects and major refactorings, alike... allowing you to "build walls" around complex functional areas, and gradually knead the framework in.
I started off using examples from the Robotlegs API and community forums, but this book gave me the extra "oomph" to tighten up my design, and learn a lot of powerful tricks that save me time every day.
I strongly recommend this book to any Flex/Actionscript developer looking to create clean, testable and low-maintenance code.