This book, like many things, was developed in response to a problem: There is no text that explains modern advancements in the field of cryptanalysis. The field of cryptanalysis has developed significantly in the last several hundred years, and for the most part, cryptanalysis has been well-studied and documented throughout this time.
However, when we move into the 20th century, the documentation of cryptanalysis has come to a near standstill. Almost every book published on the topic of "cryptanalysis" is stuck nearly 100 years in the past, idling around the area of breaking some of the simplest ciphers, by today's standards.
The field itself has not stopped developing. On the contrary, it has been moving incredibly rapidly, especially in the past 30 years, with the rise of ever more powerful computers. While all of this research into cryptanalysis has been documented and presented at various conferences throughout the world, nobody had bothered to create a simple resource with which to learn cryptanalysis from scratch. Bruce Schneier  stated that such a resource would not be worthwhile, because the field changes so much, and he has a point. But, the current roads on which cryptanalysis travels are built on the same foundations, and the amount of background material needed to understand current research or participate is becoming very large and complicated. Furthermore, the important papers are written by many different individuals with many diverse goals and audiences, ...