Lammle's stated objective is to help the reader prepare for and pass the difficult CCNA exam. This book expands a bit on that objective. He states in the introduction that this is supplementary material. It's written to assist the student in understanding the line of thought behind the technology.
I found that quite useful. The CCNA is a rigorous exam; those who pass it have a pretty good understanding of the concepts and protocols. But "real world" practice is a trickier thing, especially NAT, and Lammle's book concisely tackles those issues of keeping a network running. He clearly discusses the "why". I'm keeping the book on my desk as a reference work beyond just exam prep. It's money well-spent.