One of the complaints I've heard about this book is that it doesn't provide you with information on what different fields within a packet mean. Though the title is a little misleading, this isn't the book for that (try "The TCP/IP Guide: A Comprehensive, Illustrated Internet Protocols Reference" by Charles Kozierok).
This book is more like a very good user manual for version 0.10.0 (current version as of 4/20/06 is 0.10.14, so there are a few more features than this book covers.) What seems to be a new version of the book with a new title is due out in June of 2006, so some of you might want to wait until it gets released if you want the most up to date version.
I've been just a casual user of Ethereal for a couple years so I thought I'd learn a lot from this book. Surprisingly though, only chapters 5 "Filters" and 8 "Real World Packet Captures" were helpful to me. Everything else was either stuff I could easily figure out on my own, or things I don't use.
Except for chapter 8 with the real-world examples and possibly chapter 9, "Developing Ethereal", this book is just a user manual and should be bought only with this in mind. It would be fun if they made another book that focuses on packet analyzation using Ethereal as the tool. I'll wait.
Evaluated as a user manual only, I'll give it 4 stars. Because it's merely a user manual, it should be less expensive. Then it would earn 5 stars.