Danube Stations by J.D. Mallinson

The plot of this very original spy-detective novel unfolds during the course of a Danube River cruise, key developments occurring whenever the boat docks at Budapest, Belgrade and Bucharest en route to the Black Sea, before final denouement in London, where the story begins. The holiday atmosphere quickly takes on more sinister overtones featuring a Scotland Yard detective, ex-Nazis, the KGB and a mixed group of interesting characters. It should appeal to readers who particularly enjoy European travel and less-usual European settings.

Who is it by? :-)
Sorry, shouldn't tease!