The books intends on giving an introduction to cryptography covering both the theoretical and the practical aspects of the field.
The first two parts of the book gives an introduction to conventional (symmetric) and public-key cryptography. These parts are mainly theoretical and gives a good introduction for the novice.
The third and the fourth part of the book describes how these tools in practice. Though, it would ideally be a nice pedagogical way to teach the concepts (after all cryptography, is a lot about practice) the book fails to accomplish. The chapters mainly list the different technical aspects of the protocols, but give only few example to how the protocols work in real life. This makes it hard to use the book to get an introduction of the applications. I doubt it would helpful for a person with knowledge within the field---he or she would most likely prefer to refer to the original documents.
In conclusion the first two parts are useful for an introduction, while the two last are a pain.