We knew something about patterns and the relationships between patterns, but, this book addresses the concept of pattern language more deeply by classifying and naming those relationships, such as Complements, Compounds, and Sequences. These idea are covered in part II of the book, after talking about single patterns in part I, which I think we know more about it from the literature.
The most interesting part, from my point of view, is part III, which relates the concepts of pattern language to the well-known jargon in theory of automation. I like the analogy of patterns as words and pattern sequences as sentences, and I think we can borrow some idea of compilers to build tools that help designers that are using a pattern language. This is what I'm working as my research right now.
Finally, the book could be more compact, specially for the first two parts. But, considering the fact that the authors wanted to wrap up their ideas presented in the previous four volumes, it had to be wordy.