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:
http://www.helmet.fi/record=b1630992~S3*fin

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:

http://en.wikipedia.org/wiki/Overtone_singing
http://www.overtone.cc/songs
http://www.overtonesinging.com/
http://www.harmonicovertones.com/