First, understand that the type of clustering being discussed in this book is the statistical technique of finding clusters of data in a collection, where the collection is typically a database. This is not about clustered micro computers being used to work on big computational tasks as though it is a supercomputer.
Clusters of customers is a key area in data mining and knowledge discovery. You are usually trying to find groups of people with similar buying patterns but not necessarily identical. For instance if you have a group of people that have purchased a book on PHP, you might want to try to sell them a book on MySQL, or Apache, or Linnux. These programs fit together, but are not identical. Still the customer who purchased the PHP book is more likely to want a MySQL book than he is to want an audio CD of a murder mystery.
In this book, two of the most popular clustering techniques, K-Means and Ward's Method are presented. They are presented for a reader interested in the technical aspects of data mining as a theoretician or a practitioner. It is intended (the author says) that the material be useful to a reader with no mathematical background beyond high school. But the author also says, it might be of help if the reader is acquainted with basic notions of calculus, statistics, matrix algebra, graph theory and logic. (The author went to a different high school than I).
Clustering is described in this book to be used in a wide variety of applications, most of which are oriented to discovering social patterns, biological taxonomies, machine learning, etc. The book discusses the various techniques that have been developed and gives examples where they have been used in a wide variety of applications.