He is most well known for his 1937 book "Witchcraft, Oracles and Magic
among the Azande" where he studied the role of magic and prophecy in
the lives of the Sudanese Azande people from their perspective.
This was one of the first times that an anthropologist had attempted
to understand other cultural beliefs as a coherent system, rather than
simply listing the 'nutter' or 'irrational' practices from a Western
perspective.
One of his main conclusions was that the Azande were making rational
decisions based on different assumptions, in contrast to the general
colonial opinion that the people of Africa were somehow 'backward'.
See the documentary at:
http://www.youtube.com/watch?v=8q9HyONL_10
--DharmaTroll