The Metropolitan Museum of Art is offering free downloads of many of their out-of-print catalogues and art books here. Among them many titles on African art:
Art of the Dogon: Selections from the Lester Wunderman Collection
Royal Art of Benin: The Perls Collection
Genesis: Ideas of Origin in African Sculpture
For Spirits and Kings: African Art from the Paul and Ruth Tishman Collection
Happy downloading !