Finally got my hands on The Book! While I was very happy with the overall quality of Pentaho Solutions I have to admit that I was more anxious to get this one;
Thomas Morgner and the JFreeReport team did a very big refactor in version 3.5 of JFreeReport - codename Citrus (so big that was enough to trigger the release of version 3.5 of the Pentaho platform). And Will, as part of the JFreeReport and Pentaho development team was able to sync the release of the book with the release of the software - can't be more up to date than this.
So, not only I have a new software to play with, I also have a new book that will guide me through all the changes and improvements.
The book is not aimed to be an intruction manual on how to use reporting in the Pentaho platform, on the contrary; Will gives extensive instructions on how to use the reporting engine regardless of the environment - a java swing application, a webapplication, whatever; Just call the APIs and you'll get a report in whatever format you want. Of course, the Pentaho platform comes as an added bonus.
It does a great job explaining the structure of the engine, how to work with the report designer, how to integrate with several datasources, the export options, how to call the api and many other subjects. Fortunately it's not one of those books that waste 300 pages with the printed api, so expect to read it from start to end, probably just skipping through some (not very extensive) java code that explains how to do task N that you'll just thank to god that it's there in case you need it. Unfortunately the book can't cover everything; I was hoping to have more details on olap reporting but found only some simpler examples.
Conclusion: If you're planning to do reporting in java/webapps/pentaho/whatever just buy the book and don't complain about missing documentation for the next 2 years