I recently bought a book called "Yodel-Ay-Ee-Oooo: The Secret HItory of Yodeling Around the World" by Bart Platenga. I haven't read it yet, but here's what it says on the back:
"Yodel-Ay-Ee-Oooo" is the first book to view yodeling as a global phenomenon. It answers the question: How did a centuries-old Alpine tradition make its way into America Country music? Along the way, the reader discovers that yodeling is NOT just a Swiss thing: everyone from African Pygmies, rhinestone cow-people, avant-garde tonsil twisters, to pop starts like Jewel and Sly & the Family Stone have been known to yodel. We encounter legends Jimmie Rodgers and Gene Autry, whistling yodelers like Ronnie Ronalde, the chicken yodeling Cackle Sisters, the campy "Australian Queen of Yodeling" Mary Schneider, and the Topp Twins, a yodeling lesbian duo. "Yodel-Ay-Ee-Oooo" is the definitive deep glimpse into this secret world -- one the readr will explore with great delight.
So I guess I'll try and read the book and then get back to you on this one!