What are some themes of Extremely Loud and Incredibly Close?

As one would expect, mortality is a very important theme in the story. We all know that we're going to die one day, but we don't know when and we don't know where. This means that we somehow have to deal with our mortality, each in our own unique way; and this applies to the book's protagonist, Oskar, as much as anyone.
Oskar becomes acutely aware of his mortality in the wake of his father's death during the 9/11 terrorist attacks. What helps him to deal with this appalling tragedy is seeing his father's death from a historical standpoint, linking it to the deaths of his grandmother and an unborn child amidst the inferno of Dresden during World War II.
Yet it is no easy task for Oskar to come to terms with the uncomfortable fact of his own mortality. We can observe this when he plays the part of Hamlet in the school play. The role of the death-obsessed prince is just right for Oskar as he, like the character he plays, is plunged into existential despair: unable to discern any real purpose in life.