Who is Major Albert Knag, the Major, Hilde's Dad- from Sophie's World and what is their importance?

Major Albert Knag is a Norwegian UN soldier. He is assigned to the UN Peacekeeping Mission in Lebanon, and he wrote a book on philosophy for his daughter Hilde. Sophie and Hilde know of him first as the writer of postcards to Hilde. Later, Alberto and Sophie realize that they are characters in a book written by the Major for his daughter