This book is about piloting the plane that is your website, your program... your project. Quite simply, you don't just take off, point the nose in about the right direction and then go in back and take a nap and hope that you end up over Albuquerque in a few hours. Yet, lots of businesses plan projects exactly like that.
Usability testing is a means to checking your heading frequently and simply to make sure that your always headed in the right direction. The results of which means a more efficient development process because you know what your users want, what their reactions will be, the features that they will immediately value and the ones they will hate.
The results will almost always surprise you and contradict your assumptions.
So how does this relate to the book? The book is your instruction manually for learning how to navigate your project while in flight. It shows you how to setup basic, cheap, and informational usability studies and breaks down the reason why the extra effort turns into a HUGE cost saving method in the end.
While it is tempting to just go full-steam-ahead with a project, confident that I know the way, nearly every time I used the methods in this book to check my heading I found that I was headed in a wrong and potentially costly direction and was able to identify and course-correct quickly.
Great book Steve!