This is a very succinct and thought-provoking article which explores the origin and relationship between the Pawo (Shaman), Ngakpa (Magician) and Lama roles, outlining traditional roles with particular reference to the ancient Bonpos:

http://vajranatha.com/articles/traditions/bonpo.html

A couple of other good resources:

http://www.amazon.co.uk/Drung-Deu-Bon-Narrations-Languages/dp/8185102937/ref=sr_1_1?ie=UTF8&qid=1425297620&sr=8-1&keywords=drung+deu+and+bon

http://www.amazon.co.uk/Himalayan-Dialogue-Tibetan-directions-anthropological/dp/029911984X/ref=sr_1_1?ie=UTF8&qid=1425297798&sr=8-1&keywords=himalayan+dialogue

An interview with Khetsun Sangpo Rinpoche about the role of the Ngakpa and the role of the shaman, the differences and similarities:

http://blog.shambhala.com/2006/10/03/the-ngakpa-tradition-an-interview-with-khetsun-sangpo-rinpoche/