I finished re-reading "The Horns of the Buffalo" by John Wilcox. It's fiction but does contain fairly detailed accounts of both Isandlwana and Rorke's Drift. There are a few things that I hadn't read anywhere else, Bromhead's deafness, for example. I think that he is fairly thorough in his research. He has also written novels set in the Boxer Rebellion and the Boer War plus others, all set between about 1878 and the early 1900s. Not a great books but entertaining enough and set in a very interesting time period.