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:
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/
