Dorsai spirit / Gordon R. Dickson.

Enhanced descriptions from Syndetics:

Two cornerstone novels of the acclaimed Childe Cycle return to print in an omnibus edition--with an introduction by David Drake.
Throughout humanity's Fourteen Worlds, no group is as feared and respected as the Dorsai. The ultimate warriors, they are known for their deadly rages, and ferocious independence...and unbending honor. No one man rules the Dorsai, but their mastery of the arts of war has made them the most valuable mercenaries in the human universe.
Dorsai is the saga of Donal Graeme, the uniquely talented Dorsai, with powers beyond those of ordinary men. Once he ventures to the stars, the world of the Dorsai will be changed forever.
And The Spirit of Dorsai, written two decades later, is Dickson's great novel of the women of the Dorsai. For the warrior spirit of the Dorsai does not, cannot reside solely in the men of that race--for when the mercenary Dorsai go to their wars, it is their women who defend their home planet from the predators of the universe. And defend it they do: their home planet has never fallen to an invader, though space is full of the corpses of those who have tried.
Through three generations, the unconquerable Amanda Morgan has embodied this warrior spirit. She is more than one lone woman; she is...the spirit of the Dorsai.
In this single volume is a pair of military SF classics every fan will want to own.