This history of the Zulus under Shaka from the 60s was written by a conservative American opposed to sanctions against Apartheid. Nevertheless it seems to be a thorough study of the rise of the Zulu nation under Shaka. Is it considered a credible work by academics or is it considered tainted and unreliable?
It is extremely dated, and doesn't represent the current state of historiography concerning the Zulus. I happen to know that /u/khosikulu doesn't have a high opinion of it, and I'll repeat my charge that it looks like Morris didn't bother to consult any Zulu language sources for the book.
You'd be better served looking at more recent scholarship. John Laband has written extensively on the Anglo-Zulu war (the Kingdom in Crisis and Zulu Warrior: the battle for the South African frontier). Though, if you are narrowly interested in the period of Shaka's rule and the rise of Zulu power, those two books won't be highly relevant. I defer to /u/khosikulu's superior knowledge for any recommendations.