Journey into Fear by Eric Ambler

1940. An unassuming English engineer has travelled to Turkey on business. And somebody wants him dead.

It all began when Graham was taken to a nightclub in Istanbul and noticed a man in a crumpled suit, watching him. Then he narrowly missed being killed by gunfire on returning to his hotel room. Now, terrified, he has been helped to escape in secret on a passenger steamer home. But although Graham may try to run, he cannot hide from his pursuers forever, and soon he is caught up in a nightmare beyond his control.