The essential thesis of the book is that statecharts (a kind of finite state-machine) is not just a valuable tool for specifying user interface interaction, it is excellent for design and implementation.
What intrigues me about this is how solid the argumentation in the book is and how utterly singular this argument is. There is barely anything else written on this subject. Recently it has gotten popular to talk about state machines again; but this book was released 20 years ago!
All in all the book is an easy read and with a laser focus. Definitely recommended just for the sake of expand horizons.