In February 2015 recently published a compact method to teach overtone singing. It is a booklet of 32 pages with a CD with 54 practice examples: songs in overtones, sound poems and sining in vowels. It contains information about harmonics, describes various techniques, special effects and tips to refine overtone singing. Available in Dutch, French and German. You can order the book by transferring € 17.50 + € 5 for shipping on IBAN NL36INGB0004716967 name of Stembalans in Groet, Holland. Do not forget to clearly state the postal address on the transfer.
Sadly there is no version in english.