"The Washing of the Spears" by Donald R. Morris. The rise, history, and fall of the Zulu. All told with a wonderful lack of moral indignation for anything. Slightly more pages then you want, I think: 614 pages content and another twenty-five or so of appendices and sources, but it's worth it.
"Operation Drumbeat" by Micheal Gannon. The story, largely untold, of the U-boat war off the US Eastern Seaboard during WWII, seen partly through the eyes of the crew of U-123. Forget Pearl Harbor, this is the greatest defeat the US Navy ever suffered. About 450 pages.
"Black May" by Micheal Gannon. The climactic month of May 1943 in the Battle of the North Atlantic, and the story of the convoy ONS.5 (Outward North atlantic Slow 5), the largest convoy battle ever fought. Slightly less then 500 pages.