A Death in Vienna (Gabriel Allon #4) by Daniel Silva

The sins of the past reverberate into the present, in an extraordinary novel by the new master of international suspense.

I will not tell all the things I saw. I cannot. I owe this much to the dead. - Testimony of Irene Allon. March 19, 1957.

Art restorer and sometimes spy Gabriel Allon is sent to Vienna to discover the truth behind a bombing that killed an old friend, but while there he encounters something that turns his world upside down.
It is a face - a face that feels hauntingly familiar, a face that chills him to the bone and sends him on an urgent hunt for more: a name, a history, a connection.

Each fact he uncovers, however, only leads to more questions: each layer he strips away reveals more layers beneath. Finally, a picture begins to emerge, but one more terrible than he could have imagined, a portrait of evil stretching across sixty years and thousands of lives and into his own personal nightmares. Soon the quest for one monster becomes the quest for many. 
And the monsters are stirring . . .
Filled with sharply etched characters and prose, and a plot of astonishing intricacy and resonance, this is an uncommonly intelligent thriller by one of our very best writers.