The Moment by Douglas Kennedy

Berlin, during the period of perestroika in the 1980s. The male protagonist is a travel writer who falls desperately in love with a beautiful East Berlin woman, Petra. Their passionate relationship is destroyed when he finds out that Petra is a Stasi agent and, worse, that she has been using their relationship to gain information from him. He is devastated at such betrayal, believing that every element of their relationship had been false.

It is only years later, once Petra is dead, that the narrator discovers the truth. Petra was being forced to give information to the Stasi, who were holding her son captive under threats of death. Too late he learns that the relationship was a sincere one -- the feelings were real -- and it was only Petra's fear for her son's life that led to her betrayal of her lover. But the crucial moment, when he had the choice to commit fully to her and find the truth or to walk away, has gone for ever.

Like Kennedy's previous highly acclaimed novels, The Moment brilliantly illustrates the irrationality of love and the crucial moments which define whole lives.