Congo Mercenary by Mike Hoare
A decent, but naturally one sided, look at the activities of 5 Commando in the Congo in the mid 1960s.

WAR DOG: Fighting Other People's Wars -The Modern Mercenary in Combat by Al Venter

Soldiers of Fortune: A History of the Mercenary in Modern Warfare by Tony Geraghty

Venter covers a wide range of African mercenary operations while Geraghty covers those and more recent operations in the Balkans and Middle East.

Licensed to Kill: Hired Guns in the War on Terror by Robert Young Pelton

Corporate Warriors: The Rise of the Privatized Military Industry, Updated Edition by P. W. Singer

These two books explore the more recent phenomenon of PMCs and how they are filling a variety of roles in modern warfare.