Who is telling the story? How do you know?

The narrator is unknown and the story is told in the third-person from Bruno's point of view (for the most part). The third-person narration allows the author to switch perspectives when necessary in order to tell the whole story and express the thoughts of his characters. The novel ends....

...nothing like that could ever happen again. Not in this day and age.

From this, we have evidence that our narrator exists outside of the story.