Mask of Dimitrios

Mask of Dimitrios is a novel by Eric Ambler.

Warning: Wikipedia contains spoilers.

Ambler, a major influence on such writers as Len Deighton and an inventor of the modern "thriller" genre, is at his best in this tale of a mystery writer in Istanbul who stumbles on the trail of one Dimitrios Makrapolous, whose corpse has just been fished out of the Bosphorus by the police. The writer sets out to discover Dimitrios' past and is soon being followed from Smyrna to Athens to Sofia. Confronting his stalker, the writers receives the following proposition: "You have a piece of information which by iself is worthless; I have a piece of information which by itself is worthless; combined, however, they are worth a quarter of a million francs."