Nick Sten is a surgeon.
A man in his late 40's, at least, as he had served in the military as a physician in the Korean action twenty years before the first book in the series. He is happily married with two children at or near adult age and he is the lead surgeon at a prominent hospital in Madison, Wisconsin. Specializing in heart problems, his biggest claim to fame thus far, and the reason he finds himself involved in the cloak and dagger world is his invention of the "gas gun".
Written at least a decade before plaque build up in the arteries became a fashionable subject, but one which doctors clearly were already treating, the first novel describes the gas gun as a tool used to remove fat deposits from the arteries of living patients with very poor circulation. While Sten is credited with its invention, he modestly corrects people by saying he merely took an existing tool and altered its use for his needs. Nevertheless, it made Sten rather famous in his line of work.
It also brought him to the attention of a major KGB official, General Bogdanov, who needed someone to help restore the rapidly failing health of a key Soviet scientist. It was Sten who was formally requested to assist in this man's survival. Since the scientist was a vital player in the antimissile defense research that the Russians were undertaking, this contact of Sten also came to the attention of the CIA. The first book dealth with the machinations of both sides as one strove to keep the scientist alive while the other side wished for the other result, and Sten was stuck in the middle.
The second and final adventure deals with the disappearance, over a decade later, of a Soviet scientist in New York and the efforts of Bogdanov to find him even as the CIA goes on the same hunt. When Bogdanov suffers a major medical emergency, it is Sten that is asked to save the aging spy master's life and again the Agency has other wishes and Sten is once more caught in a place he does not want to be.
In this series, one might have chosen Sten as the protagonist but one could just as easily have picked Bogdanov because the man plays as important, if not more so, role in the adventures. Since the dramas are medical in their essence, it is Sten who was picked.