Popular anthropology, descriptive, certainly a bit dated. In fact, its idealizing picture is probably quite false and a reflection more of the author's own neurotic obsessions than of his scholarly habits.
Turnbulll, though a student of the great E. E. Evans-Pritchard, was quite an eccentric. There is a now a biography of Turnbull, by Roy Grinker, called In The Arms Of Africa.
Here is a review if Grinker, plus thebfirst chapter