I think the game design tutorial on the download page is a good place to begin. It'll take you through WED and building a couple levels. A good weekend spent with just that one tutorial should have you building levels. It does an excellent job of teaching how to place textures, lights, sound- everything.

Dan is still working on his book as far as I know. That would be a good buy as well.

After your comfortable with building levels and using templates then take a stanb at scripting. This will take a little more effort.