This is an exhaustive book that summarizes a lot of aspects of website navigation. I recommend it mostly to information architects of bigger websites. It is more detailed on navigation than the Polar Bear book (Information Architecture for the World Wide Web), though I still find that book the absolute handbook on the topic.
The book consists of three parts: The first part is about common user behaviours, types of navigation systems, labeling, etc. This part summarizes a lot of things you probably know but maybe it is not as structured in your head as the author makes it.
In the second part, the author presents a framework on how to design web navigation. From the evaluation to the actual visual layout of the navigation system, all relevant phases are discussed with examples.
The third part is about special navigation: search, social tagging and RIAs. A good intro to the topics with some interface patterns.
The book has a lot of screenshots that are quite current. Some chapters are quite self-explaining (ie. what are tag clouds, or what types of pages there are) but that is normal for a book that aims to be a baedeker for all things related to the topic.