Getting help
Through this book you have learned a lot about web mapping and GeoServer, but being an ultimate reference is far out of this book's scope.
When you are in trouble or simply curious about the new features, there are a lot of online resources that can help you. The project site, http://geoserver.org, contains a lot of information about GeoServer. Besides the basic features, you can find descriptions for the community modules that are plugins developed by the contributors to address specific requirements. Maybe you will find something really useful for you.
The project blog, http://blog.geoserver.org, announces new releases, ideas, and contributions. Your RSS feed reader can't miss it!
There are two mailing lists, one user oriented and the other for developers that are hosted on sourceforge.net. Information and links to the subscription point are at http://geoserver.org/display/GEOS/Mailing+Lists.
On both, you can ask for information about the software in general and for specific issues...