Paddy Regan is a private investigator.
He has a different name for it since that one sounded either a bit too lame or too sordid for his tastes. He and his partner, Harry, wanted to differentiate what they would do from the normal so when they started their small two-man firm, they billed themselves on their letterhead as being "special agents".
They do not have much experience at their profession when we first meet them. Regan explains early on that he and Harry had spent most of their adult life working for the British government in the Middle East as part of the "Palestine Police". When Britain relinquished its hold on that region, they were called back to England. They were offered jobs "of a sort" at the 'Appointments Office' (don't know) but that was thought by them as being far too tame after all the excitement they had been experiencing so they kindly refused and went out on their own. Again according to his own words, cases did not just immediately leap through their doors and the little money they had put away soon went further away and things started looking dicey.
Still, they stuck with it and their advertisements showed they were willing "to do anything legal" and from Regan's actions, he would have a very liberal definition of the extent of legality. Regan would also be prone to taking a small nip of whiskey what he listened to a job offer, another one after accepting the work, several more while on the job, and then quite a few afterwards.
Though I put Regan's "special agents" moniker in quotes, as it turns out, it really fits because the sort of adventures that Regan gets involved in in this three-book series written in and taking place just a handful of years after the Second World War definitely is not in the realm of keyhole peeping or dodgy husband following. And after what happens very early on to Regan's partner and almost to him more than one thereafter, that tame job at 'Appointments' might not have been such a bad idea.