If you like reading code, you probably also like reading about designs and patterns. This book is not just a description of an Emacs-like editor, and it is not just about text editing in particular.
This book is about software design and patterns that can be implemented in interactive systems in general. If you are designing an interactive system today, you should have this book digested as part of your vocabulary and design pattern library. Period.
Excercise to the reader: if you typically buy a book that explains how to use a specific language, library, or framework, you will additionally benefit from reading older sources like this. Try translating these ideas into your system of choice. This excercise includes low level details like translating from a pull-based command loop to push-based event handlers. So for example what would an event handler system look like to incorporate Emacs-like keymaps? How might this make your design simpler and more flexible for advanced users with modes and sub-editors, etc.?