H. Rider Haggard

A missionary goes alone to a Zulu-speaking people whose king answers to a witch-doctor of real power, and offers a demonstration: his god against theirs. Haggard went to South Africa at nineteen and helped raise the Union Jack over the Transvaal; the striking thing about this 1896 novel is that the wizard is not a fraud and the missionary is not a fool, and the book will not say which of them is right.