Jim Stillwater is an Intelligence Consultant.
That explanation really does not explain much but since what Stillwater does over the course of the (so far) four adventures and couple of decades changes, it is the best I can do.
When we first meet him, the time is the mid 80s during the start of the second of Reagan's two terms. He is a Lieutenant in the "Det" or Detachment, an intelligence-gathering department in the military. He was one of "Uncle Ronnie's Army", a nickname for this group, and others like it, used "for what insiders called the Army within the Army—the guys that were fighting the Cold War. The rest of the Army called them the 'Dark Side'. Journalists called them 'black ops units' and considered them to be synonymous with the CIA. The guys in these small, specialized units loved the fact that The Agency got the blame if something they did leaked out. Most of what they did in the shadows was routine. And sometimes it was just plain boring."
In the subsequent books, a lot of time has passed and Stillwater is naturally older, wiser, and perhaps less prone to willingly jump into trouble but that love of action and excitement is still alive and that gets him into the thick of things often enough.
The action moves quite a bit around the world because Stillwater's work takes him where the action is. Japan is the locale for the first, Florida (Stillwater's home) for the second, Cyprus in the third, and the islands off southern Africa for the fourth.
One important and quite enjoyable aspect to these adventures is that while Stillwater is the main protagonist, he has a varied and interesting group of solid friends who are available to help when called. Each of these people are worth knowing.