Eric Ambler Cause for Alarm (1938), Journey into Fear (1940), Uncommon Danger (1937), Epitaph for a Spy (1938), The Mask of Dimitrios (1939).
There would have to be another night of watching and waiting in the cold. He had no patience with it. If this Englishman had to be killed, let him be killed easily, quickly. A dark stretch of pavement, a knife under the ribs, a slight twist of the wrist to let the air inside the wound, and it was done. No fuss, no trouble, practically no noise. (Cause for Alarm)
I'd read these books for the gorgeously atmospheric covers alone.
These are the Penguin Modern Classics covers.
Eric Ambler is the master of the menacing 1930s' spy story. The typical storyline is of an innocent abroad caught in the evil machinations of fascists (Ambler's leanings were towards the communists in this period). The innocent is always a man who is very capable and assertive in his own field but is a complete fish out of water in the espionage game. The hero thus requires rescuing by a wiser professional spy who explains what is really going on in this exotic part of the world (Italy, Turkey, Germany, France, Greece). Often the hero is quite unheroic. Sometimes there is a lovely and clever woman on the side of the angels too, just to make it interesting.
Ambler's locations are exotic but believable (considerably more so than his helpful communists) and there is plenty of contemporary detail - politics, culture, and so on:
'My nerves', I snapped, 'are perfectly all right.'
He nodded calmly. 'That's good. You're going to need them in a minute. We're going to drop off this train when it slows down for the curve at Treviglio.'
... I couldn't adjust my mind to these new and fantastic circumstances. I found myself wondering seriously whether perhaps by pinching myself I might wake up to find that I was, after all, still in bed in Rome. But no: there was Zaleshoff smoking and gazing intently out of the window and in my pocket there was a safety-razor, a leaking tube of shaving cream, and a pair of American underpants.
These are the sort of solid, escapist reads which make good B&W films.