'Uncommon danger' was first published in 1937 (in the U.S.A. this appeared as Background to Danger; it was made into a film, Background to Danger, in 1943, directed by Raoul Walsh and starring George Raft).
Synopsis: An English journalist has lost all his money in a poker game and gets on the Nuremberg-Linz (Austria) train to borrow some from an old acquaintance. A shady character offers him a substantial sum to carry some papers across the German/Austrian border, and he accepts. When he delivers the papers in Linz, the shady character has been murdered, and our English journalist is accused of the murder. He has to find the real murderer, as well as what the papers represent, and gets into deep political waters.
This book is amazingly different from Ambler's first book, 'Dark Frontier', from 1936, which didn't quite know if it wanted to be a pastiche or a thriller. I would say this one is the first of modern 'spy' thrillers, the forerunner of Deighton, Le Carre et al. There is a really good atmosphere of increasing oppression, the 'hero' is all too human, confused and scared, but stubborn and opinionated too. A nice touch is that his helpers against the forces of evil are.. communists. But then the book was published two years before the Molotov-Ribbentrop pact...
The book has a very nice line in irony and cynicism, but can be emotional too (the outburst of the English commercial traveller comes as quite a shock), and deals, in a very Ambler-fashion, with the reactions of a normal person, an amateur, with high politics and big business, and the tentacles of 'policy' carried out by criminals.
A surprisingly modern story, really; and still very readable, and still very enjoyable.