The book is catalog of pattern illustrated by pictures and few lines of text.
It's useful to understand the various existing patterns. Adopting well known design pattern make the life of your users much simpler. At the first sight they understand how your app works and its navigational scheme.
The book is also useful in labelling with names the patterns. This helps you in referring correctly and consistently to them. Names also help you in identify and recognise the patterns while you use apps made by others.