Dennis Wheatley's 1936 novel Contraband is a strong and concentrated example of the inter-war UK thriller. It features Gregory Sallust, who, for want of a better word, we can call a freelance problem solver.
In Contraband he is employed by magnate Sir Pellinore Gwaine-Cust to uncover a network smuggling silk from France to England.
Sallust gets thrown in with Inspector Wells from Scotland Yard, and together they turn up allies and new enemies at every turn. The smugglers use air planes, fast boats, hidden landing fields, and caves around Romney Marsh, Pegwell Bay, and the Isle of Sheppey to make their deliveries by moonlight.
There are several solid and sustained action scenes at night, featuring cat and mouse pursuit and some shocking reversals. Their chief enemy is the younger brother of the Duke of Denver,Lord Gavin Fortescue:
....He's the Duke of Denver's twin, but whereas Denver is a fine upstanding figure, Gavin is a sort of freak. Not a dwarf exactly but very short, with an enormous head and a tiny little body, like a child's in its early teens. They say that his abnormality together with the fact that he was born second, and so failed to inherit the dukedom, embittered him to such an extent that it turned his brain. The story goes that he even attempted Denver's murder when they were boys together.
(Readers of Dorothy L. Sayers' novel Clouds of Witness will recall a different Duke of Denver, and a very different younger brother: Lord Peter Wimsey.)
Sallust is superior to Bulldog Drummond, whose adventures I have also been reading. There is no casual pleasure at the prospect of bullying and punishment. And the n-word does not make an appearance. Whereas Drummond is a vigilante contemptuous of the forces of law and order, Sallust is a professional hired to do a professional's work.
Wheatley gives us two or three sentences suggesting Lord Gavin is part of a communist plot to ruin the UK economy and import outside agitators. Anyone familiar with the class-collaborationist politicians of the Comintern circa 1936 will be amused by this. By 1936 anyone abdication proletarian revolution had already been expelled from the Communist Party of Britain as an anarchist or Trotskyite. Class collaboration and Popular Front were the order of the day.
I'll close with an excerpt from chapters 16 and 17. Sallust and Wells have been kidnapped by Lord Gavin's men, and must fight for their lives against the sucking tidal sands of Pegwell Bay.