The Defector by Daniel Silva

Spy turned art restorer Gabriel Allon is in Umbria, when shocking news arrives from London. A Russian defector who saved his life has vanished without a trace. British Intelligence suspect he was a double agent, but Allon knows better. Soon he finds himself in a deadly duel of wits with a murderous Russian arms dealer as he bargains for his friend's life.

Criss-crossing Europe at breakneck pace and culminating in a heart-stopping climax in the freezing birch forests of Russia, The Defector is a searing and bloody tale of courage and vengeance