Quite a number of helpful manuals can be found at the Web page of PRAGMA ADE. Some have some very interesting designs. I wonder, is there a place to download the source code for some or all of these manuals?
The source code for ConTeXt beginner's manual and the revised reference manual are available at http://foundry.supelec.fr/gf/project/contextman/scmsvn/.
The source of most other manuals can be browsed at http://context.aanhet.net/svn/. There is a svn repo for other manuals as well, but I cannot find its address.