Could you help me to find overtone singing in digital libraries?
I searched from Helsinki City Libary's database and found only one book on overtone singing. The book is called "Overtone singing: physics and metaphysics of harmonics in east and west" and it is written by Marc C. van Tongeren:
Wikipedia has a very fine article about overtone singing. The article has an excellent list of external references. Many of these refenrences have audio clips.
Overtone singing links: