Polyphonic Overtone Singing: an acoustic and physiological (MRI) analysis and a first-person description of a unique mode of singing 
Written by Anna-Maria Hefele, Robert Eklund, Anita Mc Allister, 2019
Link to official publication HERE

Obertongesang- eine besondere Stimmtechnik als Kunstform 
veröffentlicht in: Vox Humana, 01/2017

Western Techniques of Overtone Singing and their Application in Classically Notated Music 
published in: International Choral Bulletin, 01/2015

Westliche Obertongesangstechnik und ihre Anwendung in der klassisch notierten Musik
veröffentlicht in: International Choral Bulletin, 1/2015

Eine neue Dimension für die Stimme
von Eva Runkel, veröffentlicht in der Süddeutschen Zeitung, 19. 10. 2018

Voice ranges of Anna-Maria Hefele in overtone singing
green notes work well, red ones are impossible, orange does work, but doesn’t sound that good.

Here are more resources about overtone singing available to download for free on Wolfgang Saus‘ webpage, including voice ranges for male voices.

The software I use to make sound visible is called Overtone Analyzer by Bodo Maass, it can be downloaded HERE

Very interesting is the DVD „The Voice“ by the institute for musician’s medicine, where I participated as overtone singer for MRI recordings. Published at Helbling in 2017, available HERE