We bought this book in my company to extend Eclipse with our own plugins to support our own software framework.
To use it, you really have to sit down and work through every chapter, but sometimes you have to skip a lot of certainly useful, but not relevant at that point, information.
This way it falls between the chairs of being a tutorial and a reference.
Eclipse is developing fast, and of course this only covers 3.3 and 3.4, where Eclipse is currently at 3.5. The information is still usefull, but you have to make allowance for the differences.
Also, a few things are missing. E.g. to layout things properly, you have to call layout() or pack() on your SWT composite graphical objects, when you change it. This took me a long time to figure out, as it not mentioned, not even in the sample code.
Also, the downloadable code is updated to support the newest Eclipse version, but still even the original code is not in sync with the text.
On the other hand, we have been helped along tremendously, the Eclipse plug-in developement environment may be documented well at the API level, but we have not found any tutorial introduction other than this.