Behind the mask
In the case of Dimitrios, the situation may have been exacerbated, as I remember the film very well, and thoroughly enjoyed it. A classic film noir, with a great cast including Sydney Greenstreet and Peter Lorre. I don't think the book is one of Ambler's best, but on reading it properly (I don't think I ever made it to the end before), there is much to commend it.
First of all there's some great (and influential) charcterisation. The hero, Charles Latimer, a writer of crime fiction unexpectedly thrown into the sordid world of drugs-dealing and international crime, must surely have been the prototype for Graham Greene's good-natured western writer, Holly Martins, thrown into the world of Viennese spivs, and the deadly but charming Harry Lime of Third Man fame.
It's also a great storyline, which, again, Greene re-used to a certain extent in The Third Man. There is a great sense of place as Latimer moves across Europe in pursuit of the background to the body in the morgue. Dimitrios centres around the crime-writer, Charles Latimer, who while on holiday in Turkey befriends the chief of police, Colonel Haki (possibly an inspiration for the friendly Turkish police chief in the film of From Russia with love). Haki invites Latimer to a morgue to see the body of villain, Dimitrios. Latimer decides to trace Dimitrios' path across Europe, wondering what had turned him into a master criminal.
The mask of Dimitrios gently pokes fun at the "cosy crime" of the English Golden Age of Detective Fiction, as Latimer will ultimately turn away repelled by the violent world of Dimitrios, and sink back into his own apparently violent but actually secure world of the imagination. Although set against a Europe that is rapidly being overtaken by Fascism and sliding towards war, Dimitrios is a classic thriller of the old school. There's no escaping nasty Nazis here, this is about violent crime, which changes little irrespective of governments; though 1930's attitudes towards drug-taking do seem rather strange, at least to the sensibilities of someone born much later in the century. Perhaps it's because it has little to do with its political background that I failed to be enthralled by Dimitrios in the way that I was with Uncommon Danger or Cause for alarm.
Nonetheless Dimitrios is well worth reading, if only to realise how ahead of the game Ambler was in the 1930s. For a first Ambler though, I would definitely recommend the more political thrillers.