Other Links Old-Time Books carries books specifically about classic country music. Bart Platenga's essay on yodeling. An overview of yodeling in Inside Now magazine which is interesting in that it doesn't focus exclusively on one type. Charged has a fun article on yodeling. It provides karaoke background for yodeling.
Space Ghost gives advice on yodeling.
This appears to be a children's program. The Institute of Totally Useless Skills lists yodeling. Unfortunately, things are probably going to be mothballed for a while due to a hellacious flareup of Repetitive Strain Injury. There are some folks I would love to keep in touch with, but whose addresses I've lost.