A very informative book with some great field-photos, available for free here (20 MB, file info).
As read here, more old publications on African art are downloadable in pdf-format on the website of the Cultural Heritage Library of the Smithsonian Libraries.
We’ve been building this Congolese collection at the Warren M. Robbins Library for 33 years and are pleased to be able to share it with scholars and students globally, especially those living and working on the continent of Africa, where none of these books are likely to be found. Of particular interest is the set entitled Congo illustré, which contains historical photographs of Congolese society, Belgian colonial & missionary enterprises, and flora & fauna.
Their database can be searched here.