Warhammer 40000: Space Wolf
Warhammer 40K novel, one of the first that showed the selection and training that goes into the making of a Space Marine (outside of the fluff in Codex Astartes).King picks an interesting Chapter and the people who make it what it is, with plenty of action from the start.
It follows Ragnar, starting as a primitive youth and then being chosen and trained to become a Space Marine. Good descriptions of what is going on and the changes it brings on all the aspiring initiates.
