A tale from experience...
This book takes the interested beginner through all the steps involved in creating extensions for the Eclipse platform (www.eclipse.org). As part of my research I am extending Eclipse, and spent the first six months of this work hunting high and low for threads of reasoning which might help me.
The Eclipse platform is relatively young, and open-source: consequently definitive discursive documentation is hard to come by. The eclipse.org site is good, but does not contain any full, considered, end-to-end examples.
In contrast, this book teaches by example, and explains the decisions along the way. The examples are taken in "circles"; showing how the code begins with Eclipse extension points and builds until it too is capable of being published and extended.
Much of the knowledge which I had to fight hard to obtain in that first six months is covered in the first circle - this was hard won understanding which had taken many hours of reading non-relevant or incomplete information. To have all those half-truths, mis-understandings, possibilities and uncertainties cleared up in just a few chapters made me significantly more productive and more effective.
I read several of the early drafts of this book and based on those, I've pre-ordered my copy. If it turns out to be a dud, I'll amend my recommendation, but based on how much it helped me so far I can't give it less than five stars.