In case you are still looking for some summer reading material, Seven Years among the Fjort, Being an English Trader’s Experience in the Congo District, published in 1887 by Richard Edward Dennett (freely downloadable here) might be an interesting suggestion. Dennett was an English trader who left the UK for Congo in the employment of Hatton & Cookson, who mainly imported ivory, in 1879. The above book, published in 1887, recounts the seven years he spent among the Kongo-Vili (then known as Fjort). In chapter III (pp. 46-72), he discusses nkissism and on page 66 we find one of the earliest descriptions of a nkisi nkondi. Facing page 48, we find the below photo of Feteiches and Curios. Raoul Lehuard translated the book in French as Sept ans parmi les Bavili: Les expériences d’un marchand anglais dans la région du Congo (Collection Arts d’Afrique noire, 1991).
In 1889, Dennett donated over 70 objects collected by him to the Royal Albert Memorial Museum’s World Cultures collection. You can see some of them here – with such an old provenance definitely worth a look !