James Nathaniel Pettigrew is an agent with the British Secret Service.
As he explains quite clearly in the first chapter of the first book, the agent was pleased with the name as it sounded "harmless" and it was for that reason the name had been chosen for him by the Service. What his real name is never gets mentioned in the three-book series which take place at the end of the 1950's. The name was given him some time after he started work with them and he has kept using it because of the cover that he had established with it.
To the world outside his department, Pettigrew was an accomplished writer of travel-books. He has been doing it for some time and making a good living at it and, more importantly, travelling to a considerable number of places to pen the spectacular wonders these places can show, letting the readers know what they would see should they go there and how to best enjoy things when there. For those who would never actually make it, his books partially filled a void and gave the reader some idea of the sites and sounds. And while at these places writing about these beauties, Pettigrew was able to do his real work as agent in the British Intelligence community.
Back to his name signifying his being harmless, Pettigrew was quite proud of the fact that his work at being virtually invisible paid off so well. He describes himself as "medium build; quietly dressed; indeterminate features." As he put it, he was the guy that stood next to you on the subway, the guy you never noticed or remembered, the guy of no importance. Still according to Pettigrew, he was not always so plain and ordinary and forgettable - this had taken training and practice but now it was second nature to him.
Exactly how long Pettigrew had been an agent is not given but it was sufficiently long to earn him the positional title of "Senior Agent" which gave him the right to make decisions on the spot, not needing to always check in first. His experience also gave him the ability to recognize quickly how good or bad other agents were and on whom he might be able to count when things got bad. To his sadness, the number that could be relied on were few.
Such was James Pettigrew for the first two recorded adventures.
In the third and final book, Pettigrew was no more and the man who had been Pettigrew now had a new face because his old one, and identity, had been discovered by the "opposition" and in the place where Pettiegrew might have been was Charles Graham, a retired schoolmaster who was now writing "nature notes" for a major London newspaper. Though he looked differently and answered to a new name, he was the same man as was never really Pettigrew.