The Ambler novel is not a mystery; it is a caper novel. What makes it interesting for me is how Ambler inverted it with his main character. I have not seen the movie yet - it came out in the early 60s
Ambler was a screenwriter as much as he was a novelist. It would be interesting to see how the screen treatment of Arthur Simpson varied from the Ambler novel - Ambler did not write the screenplay. I wonder how Peter Ustinov played Simpson.