It's everything you need to know, from the bottom up.
Let me first explain a little about myself. I wrote my first program in 1982 on a System/34 (RPG II). Two years later, I was on the System/38 (RPG III), and soon after, the AS/400 (RPG IV, etc.). So, yes, I am an old green-screen developer—working hard, I might add, to update my skill set using Rational Developer for i for SOA Construction (RDi SOA).
I found Joe's book very useful in introducing me to the key concepts of EGL—namely widgets, library functions, reusable components, and completely encapsulated shared projects.
This book is as advertised: a working introduction to the EGL Rich UI technology and RDi SOA tooling, showcasing RDi SOA's ability to quickly build rich graphical applications for the IBM i platform.
In the book, you begin slowly, adding widgets via the WYSIWYG editor, changing box properties, adding box properties, etc. You advance to packaging applications and managing error-handling and testing. The book does a particularly nice job in explaining RDi SOA's ability to let you debug an application across multiple platforms, using a consistent end-to-end debugging environment. The book also explains how to use RDi SOA to debug both EGL and ILE components, from within a single workbench.
In summary, I have found this book to be an invaluable tool in improving my EGL skill set. It is no wonder Joe is one of the more sought-after speakers year after year at System i conferences.