Marston Baines is an agent with British Intelligence.
That is, not surprising, not a very well known fact since his success relies in large part to his anonymity. Because, however, he has to do something to make a living, he can claim considerable success from being a thriller writer. This profession not only gives him the official funds to travel as he wishes, it gives him the time and the incentive - new locales bring new ideas for stories.
As a result, Baines can go to a variety of places and, if noticed, find that acknowledgment as a "oh, he's just a writer".
Baines has a nephew with whom he is quite close. Simon considers Uncle Marston to be almost a second father and when the opportunity arises to travel with his uncle, well, any break from university is a welcome one and Baines goes to many impressive locations. Young Simon is, at first, not aware of what his uncle really does for a living but being a very inquisitive and quite bright individual, it is not long in their journeys before he gets a whiff of the truth and digs further until he knows the facts.
When they first meet after Baines invites the young man to spend some time with him in his small villa in Tuscany after the death of Simon's father, Simon is not overly impressed with his uncle. An early passage states that "Marston Baines was between forty-five and fifty; stoutish but loosely built, clean shaven, with thinning hair and untidy in appearance." Further, "Simon wasn't sure what professional writers looked like, but his first impression of his uncle was that he didn't look like anybody very significant."
The series really revolves around Baines' nephew, Simon, and his frequent accompanying friends who, once Simon learns the real work that Baines does, unofficially joins in and lends a hand. Simon has no formal training in the tradecraft but he picks up things pretty quickly from his uncle and he proves more than once he is a useful resource, when he is not getting himself into a pickle and needs helps getting out.