The format — aside from some introductory remarks and their (pretty good!) breakdown of how they categorize the different methods— is a general overview of a method and where it comes from. This tends to point you in the right direction if you want to learn more about it or employ it yourself.
To be honest, I have not read this book cover to cover (though I've probably tried on more than one occasion). It's a good reference: 1 method per 2-page spread. Writeup and references on one side, illustrations/pictures/diagrams on the other. On the other hand, there isn't a thread pulling you through the whole book. Perhaps one criticism (If I remember correctly, maybe I don't) is that I wish methods in the table of contents were ordered differently.
I've found it useful when I needed a bit of inspiration by reviewing methods I haven't used in a while, or to learn about something entirely new. If you want to follow a method to a 't,' and not as something for inspiration or remixing into something else in design research, there's a good chance you'll want to do more digging or research for advice on how to prep and conduct out the method.