Gordon R Dickson 
Soldier, Ask Not 
The Childe Cycle Book 3

The black-clad mercenaries of the Friendly planets fought where their employer and their God dictated. On New Earth they pitted their fanaticism against the cold courage of the Dorsai.And the implacable hatred of one man, Tam Olyn. Olyn saw his brother-in-law shot down before his eyes. His quest for vengeance took him across half the civilised worlds, to Cassida and Frieland, to St. Marie and back to New Earth. He met men of all the splinter groups into which mankind had evolved an he used them all to bring about his revenge – until Padma the Exotic taught him how to use his special powers, and the frightening knowledge of Final Encyclopaedia.