As the book settles down and gets past the half way mark you are juggling three time periods. The life of the grandfather who shortly after his marriage was to face the Nazis sweeping through the village. The current day search for the story of his escape and the interaction between the interpeter and the author in letters written at stages commenting on the discoveries made as they search for the truth about the past.
Part of the problem is that before any of the stories really opens up and grabs you the time shifts and you go forwards and backwards. The past does inform the present but it does feel sometimes as if the action has not been given the chance to breathe.
You know that the intertwining of the different periods is going to be key to the book and the structure is as important as any of the main characters but perhaps the reaction to that is the reader's choice. You either love it and view it as being very clever or you see it as perhaps over worked and a structure that breaks the continuity.