Between 1965 and 1996, Elleston Trevor wrote 19 novels under the name Adam Hall featuring his character, Quiller, a British intelligence operative, who worked for the "Bureau." The novels are:
The Berlin Memorandum (1965)
The Ninth Directive (1966)
The Striker Portfolio (1968)
The Warsaw Document (1971)
The Tango Briefing (1973)
The Mandarin Cypher (1975)
The Kobra Manifesto (1976)
The Sinkiang Executive (1978)
The Scorpion Signal (1979)
The Pekin Target (1981)
Quiller/Northlight (1985)
Quiller's Run (1988)
Quiller KGB (1989)
Quiller Barracuda (1990)
Quiller Bamboo (1991)
Quiller Solitaire (1992)
Quiller Meridian (1993)
Quiller Salamander (1994)
Quiller Balalaika (1996)
There was also a short story titled Last Rites, written in 1986.
The stories are written in the first-person and follow Quiller from initial briefing of his mission to the end of the mission. These are not Executioner-style men's adventure books. There is a fair bit of action, but also a considerable amount of introspection. They are closer to Fleming than Le Carre, though I would personally rate the Quiller novels as being better than the Bond novels of Fleming. You really get to know how Quiller is feeling: his fears and anger that the Bureau could cast him adrift at any time if it suited them are an important thread in the novels. In fact, in one mission the Bureau rigs a car bomb to kill Quiller in the middle of a mission when he becomes expendable.
All the novels are recommended if you like taut, suspenseful thrillers, with believable plots and action. Oh, and one thing more, Quiller never carries a gun, preferring to rely on his considerable self-defence skills.
The film, The Quiller Memorandum, starring George Segal, was based on the first Quiller novel , The Berlin Memorandum, and filmed in 1966. There was also a short-lived BBC series produced in 1975 called Quiller and featured Michael Jayston as the titular character.