Rex Carver is a part-time agent for the British Secret Service.
His official line of work is that of a private investigator. He doesn't want to be anything else and, when he has a few pounds in his pocket and a few more in the bank, he doesn't even want to be that. But an investigator he is, working out of London, in a one-man agency with just his secretary, and minority partner, to help.
Carver does, though, on occasion, work with Richard Marston, a senior agent with the British Secret Service. The work is usually his normal type of work, finding people or things, but when Marston wants his help, there is usually a lot more behind the case than anyone is letting on.
Carver is a good enough detective to dig deep into the confusion to come up with the truth. As a detective, he gets into plenty of fights with those who don't want the truth known. As a sometime spy, those people hiding the truth tend to do so for political or patriotic reason instead of the seedier motives one might expect. But they shoot as deadly and fight as vehemently. Being shot at is not something Carver appreciates. Unfortunately for him, it happens far too often.
The Carver adventures are post-noir in that the detective is not hard-boiled so much as just plain cheeky, to use a British expression. Whatever the term might be, Carver has little regard for authority. He also has little respect for those better off than he and not much for those worse off and hardly any for anyone else for that matter. He would call himself a cynic if he thought he could be trusted.
Note: There is a prequel of sorts, marked as Book 0 in the list. This book was written by Mr. Canning two years before the first Carver adventure and tells of a fascinating adventure the supporting character Marston had himself. Because Marston plays an important, albeit minor role in the Carver novels, it is included.