Inspired by the Merb Internals Handbook, the Grails community has begun working on a handbook of their own: The Grails Internals Handbook.

From the README:

If you want to contribute, fork this repository and go ahead. Pull requests on GitHub are more than welcome. If you use patches, make sure you use git format-patch and your patch has author name and email address on it.

We love to see this stuff – keep ’em coming, and good luck!