The Old Enemy — Henry Porter

The ex-MI6 agent is on the run as he uncovers a deadly conspiracy reaching back to the days of Cold War espionage in this acclaimed thriller series.
 
Former MI6 agent Paul Samson is shadowing a young woman around London for a private security company. Though the brilliant Zoe Freemantle is intriguing, the work is a bit dull -- until Samson is almost killed by a thuggish assassin. When other people connected to Paul come under attack, including legendary spy Robert Harland and billionaire Denis Hisami, Paul escapes to Estonia to make sense of the mysterious threat.
 
Paul knows there's a target on his back. The only question is whose finger is on the trigger. Together with Denis's wife Anastasia, Paul picks up the trail of a former Stasi agent whose network of assets go deep into the US and UK governments. Now, Paul and Anastasia must expose the spymaster before any more people are killed or agencies compromised.
 
An astonishing and timely thriller examining the penetration of Russian assets into all levels of western life, The Old Enemy is a complex, breathtaking race against time from “one of our most accomplished thriller writers” (Financial Times).