The Messenger book by Daniel Silva

Gabriel Allon, art restorer and spy, has been widely acclaimed as one of the most fascinating characters in the genre and now he is about to face the greatest challenge of his life.
Allon is recovering from a grueling showdown with a Palestinian master terrorist, when a figure from his past arrives in Jerusalem. Monsignor Luigi Donati is the private secretary to His Holiness Pope Paul VII, and a man as ruthless as he is intelligent. Now, however, he has come to seek Allon's help. A young Swiss guard has been found dead in St. Peter's Basilica, and although Donati has allowed the official inquiry to determine that it is suicide, his instinct tells him that it is murder-and that his master is in grave danger. He has trusted Allon in the past, and he is the only man he trusts now.Allon reluctantly agrees to get involved, but once he begins to investigate he concludes that Donati has every right to be concerned, as, following the trail from the heart of the Vatican to the valleys of Switzerland and beyond, he slowly unravels a conspiracy of lies and deception. An extraordinary enemy walks among them, with but one goal: the most spectacular assassination ever attempted.