Creating A Dungeon
One thing I have always thought that the books don't cover well enough was providing more resources to aid in creating dungeons. There is the simple map key Gary Gygax provides in the DMG and that is a good start, but, more can be made available.
Some common symbols used for maps are necessary to creating a quality map that any DM/GM can understand. I am collecting as many symbols and resources to help other would be "hand made" map makers to accomplish this task.