Geoffrey Landon is an agent for the Organization.
This formally unnamed branch of British Intelligence is stated as being at the lower end of totem, the part seldom seen or appreciated. As a result, it gets the very much unglamorous jobs and, worse yet, very much the short end of funding.
As the series opens, Landon considers he is only a part-time agent, though this changes by the second book. He is a former musician whose career was cut short by military obligations. After six months as a private just performing standard tasks, his other talents were noticed and he was pulled into Intelligence. A promotion to Lieutenant and work more suitable for his intellect followed but so did the danger and at one point he was forced to go into hiding for many months as a result of an assignment gone bad.
When he finally had enough and got out, he went back to the music world. There, however, he found his classical talents were not up to what was required in the concert arenas and he lacked the drive and ambition needed to get it back. Still needing to make a living he did the next best thing: he became a critic. Now he wrote columns and reviews and even had an occasional radio program.
This allowed him a small income but also the chance to travel about Europe writing about concerts and orchestras and bands and the music scene in general. This, naturally, brought him to the attention of the Organization who needed someone who could travel easily and with little notice.
The cover for the Organization is Portland Productions, Ltd under the command of a man named Sawyer. The agents who work for Sawyer are on no personnel list and do not work towards any pension or expected retirement. Such matters are usually not needed as they don't live long with the assignments they are given.
In Landon's case, the jobs that he typically did were very low key and uneventful. At least, that's how they started.