Trying to identify the story; it's in 4 or so parts (I would guess published as a serial), and the hero is some form of space-age Spy.
Spies of his nature were EXTREMELY undercover, usually, are given memory implants of specific people (either who they are seeking, or someone they are imitating), and pseudo skin and such, as needed. They are conditioned to respond as the person they are done up as, this conditioning only breaking down under extreme conditions. They are refered to as 'Prime-Operators' or some such; if one should die or go missing, an entire planet could be faced with extinction as a reprisal.
There are several stories I recall; the first involved aliens that, after something was done to them, lived at an exteremely slow pace -- the horror came when a human had this done to him at the end, after being stabbed. (The hero was wounded, but lived.) The plot involved some action being taken by the people running the facility on the planet that was resulting in excellent output, but at the cost of the natives dying quickly, due to a powder they were being dosed with via their barefeet and the floor of the mine.
Another story had the hero taking the personality of a man who was a member of the nobility on some planet, but this one broke down quickly due to unforseen circumstances. He spends a chunk of it in jail/prison and takes a leave afterwards with the women he was stuck with.
In the end, the hero was sufficiently conflicted by all of the horrific things he had to do, that they end up wiping his mind or killing him at the very end.
The cover of the paperback copy I once had was of a man, on his knees, leaning back in horror, as various masks litter the ground near him. Based on that description, searchings keep bring me to 'Today We Choose Faces', which, although a good story, is NOT the one I'm looking for.