That's hard for me to say really, because although I have most of his Osprey books and of course the NAM one I have not read the book which, after looking around, I consider to be the closest thing to it thet he has written. I am talking about Brave Men's Blood, which I think is part of the coffee table brigade. Given the review Knight gave Saul David's Zulu on the Zulu War Research society website it was probably written to give it a bump off the shelf, but that's just my opinion.
However if it helps, my reading of Zulu rising will not stop me from eventually buying Brave men's Blood. Centering on iSandlwana and Rorkes Drift, so far it blocks together the sequence of events very well and brings to life characters from both sides in a very engaging way, adding nice depth both to the men and the story. Having yet to finish it, I will go out on a limb and say that at the very least you could call it a summery of all he has written about before in one volume, it being something of a culmination of nearly 30 years experience in the field of the Zulu War, so I think it's safe to say you might find a few new things in it.
Really the Bottom line is, that if you like the Zulu War you will want to buy it. I still want his other books. For instance I have read extensively about Waterloo and could probably talk about it from end to beginning, but should another book like "The Battle" come out I would certainly snap it up. Zulu Rising I think is one of those sort of books, I did hold off getting it for a while but then eventually succumbed (Who was I kidding anyway!) and I am enjoying it immensely).
Hope I have helped rather than hindered, will let you know when I finish.