I just finished a rough draft of a 20 page tutorial on GIMP paintbrushes, free at: http://oneoffpressbooks.com/pages/gimp.htm
This is an excerpt from GIMP Supremacy. The intended audience, when I started the project, is the non-digital artist who has a need for an image manipulation program. In trying to be thorough, the project has grown a bit.
Feedback is more than welcome.