Mockler, a British journalist, opens with a concise summary of mercenary warfare from Xenophon's Ten Thousand to the exploits of the French Foreign Legion, then focuses on mercenary activity during the last quarter century. Most of this has taken place in Africa, led by such infamous mercenaries as ``Mad Mike'' Hoare of South Africa, Frenchman Bob Denard, German Rolf Steiner and the ferocious young Cypriot ``Colonel Callan.'' Though these potentially interesting characters come across as two-dimensional, the narrative itself is rich in mercenary lore, with details of recruitment, political arrangements, field campaigns and the 1976 show-trial of Colonel Callan and his men in Luanda. Readers will find some of the action nightmarish, especially the accounts of internecine slaughter between mercenary groups. Mockler virtually accuses Frederick Forsyth (The Day of the Jackal, The Odessa File of planning and financing a coup in Fernando Poo and writing a ``factional'' account of it called The Dogs of War. Photos. (May 15)