I know specialist in mercenary soldiers was The Dorsai...
from Wikipedia:
Dorsai! is the first published book of the incomplete
Childe Cycle series of science fiction novels by American writer
Gordon R. Dickson. Later books are set both before and after the events in
Dorsai!.
The novel was originally published in serialized form in
Astounding Science Fiction, starting in May 1959. A shorter, revised version was published in paperback by
Ace in 1960 under the title
The Genetic General. A re-edited and expanded version of the novel was published under its original serialized title,
Dorsai!, by
DAW in 1976. This version of the novel was reissued as one half of an omnibus edition,
Dorsai Spirit by
Tor in 2002. The other novel contained in
Dorsai Spirit is
The Spirit of Dorsai (originally published 1979).
The book is about Donal Graeme, warrior extraordinaire. In the Childe Cycle universe, the human race has split into a number of splinter cultures. Donal is a member of the Dorsai, a splinter culture based on the planet of the same name, which has specialized in producing the very best soldiers. Since each splinter culture specializes in a specific area of expertise, a system of trade labour contracts between the cultures allows each planet to hire the expertise they need. The Dorsai, inhabiting a resource-poor world, hire themselves out as
mercenaries to other planetary governments. Donal has great ambitions, and the book follows his rise in an episodic manner. The book begins as a straightforward tale of his career and then becomes something else, as it becomes clear there is something different about Donal Graeme himself.
