Final answer:
While the provided source focuses on Kongo religion, works by scholars like Placide Tempels and follow-ups by John Mbiti and Alexis Kagame would offer insights into Bantu philosophy, which may include Zulu beliefs in a sky deity. The concept of ubuntu and broader studies on African cosmology are also relevant.
Step-by-step explanation:
A key source documenting Zulu belief in a sky deity is the research and writings of scholars who have studied African religions and philosophies. While the provided reference to JANZEN, John and Wyatt MACGAFFEY is invaluable for understanding Kongo religion, it does not directly address Zulu beliefs. Instead, works by scholars like Placide Tempels, who published Bantu Philosophy, and African scholars such as John Mbiti and Alexis Kagame would provide a more authentic examination of Bantu philosophy and, possibly, Zulu deity concepts.
Tempels highlighted the Bantu belief in a 'vital force' that originates from God, and later scholars like Mbiti expanded upon these ideas. The Zulu term ubuntu, emphasizing interdependence among people, gives insight into the communal aspects of Bantu and therefore possibly Zulu spirituality. Additionally, the works of Mutombo Nkulu-N'Sengha and Cheikh Anta Diop can offer broader perspectives on African deities and cosmology. These sources together may offer documentation and understanding of Zulu theism within their spiritual and cultural context.