The Janus Man:

The story concerns treason and the rooting out of a traitor and double agent among members of the Secret Intelligence Service, also known as the SIS or MI6. The protagonist is a man known as Tweed, and the book follows his efforts to investigate the clumsily disguised murder of colleague Ian Fergusson, dubbed an "accident" by authorities, whilst in Hamburg, Germany, and the reasons for his killing. Circumstances lead him to believe that the only way the enemy (the Soviet Union) could have intercepted Ferguson on his mission would have been to have a double agent stationed within the SIS. As he attempts to discover the identity of "The Janus Man who faces both East and West", he tracks sources of information in Moscow, Lübeck, Copenhagen and Oslo to hunt down the killer of Ferguson.