The first section of the book, called "Guiding Principles", lays the foundation to begin thinking about usability. It explains the thought process that the end user takes; it's never what you plan, but with proper design, you can help guide and funnel them in the right direction.
I've explained it in another post before: developers often think exactly like their application: we think, "if the user clicks X, they'll see a menu for Y, which they can click to get to Z. Wire it up!" but in reality, the user isn't a developer and doesn't know the application end-to end. They have to muddle their way through the website, and if they're lucky, they'll manage to follow your carefully laid track. The only way we can have a hope to get them from X->Y->Z is by making each link, button, or step as clear as possible by developing controls that are self-explanatory in use. Buttons should look clickable, links should stand out from the text, headers should be obvious, and navigation should be clear. The user shouldn't have to look up help text, experiment, or even pause to think in their quest to get to Z.