The title of this book is a bit misleading. It's really an update to Jonathan Hassell's Learning Windows Server 2003 yet O'Reilly has decided to put it under their Definitive Guide series. Some of the chapters are almost exactly the same as the '03 book.
This is not necessarily a bad thing, as I felt his descriptions were solid enough to get a general idea and in these areas MS hasn't changed much that the admin would notice. And if you didn't read the previous book, it is a good idea to give these sections a read anyway. Some major changes in '08 are called out, but it still helps to skim the text. And if you didn't read the previous book, it is a good idea to give these sections a read anyway. Even in areas where the material is minimally changed, some of the examples and diagrams are updated and made clearer.
Server 2008 features such as IIS 7, PowerShell, and Hyper-V are given their own chapters which give decent introductions to topics which rightly have their own books.
I still suggest having a much larger and hopefully searchable book on standby when you finish this book and start putting the OS to work. I would also suggest separate books on AD, Group Policy, IIS, etc as necessary. Why not just buy those thick books to begin with and skip this skinny 450 page book (150 pages slimmer than the previous edition)? It's all about top-down thinking. I like having an accessible introductory text that will let me start tinkering on my own and build context before going into a deep dive.