Extremely Loud & Incredibly Close

I'm really enjoying Jonathan Safran Foer's writing style from what I've read so far in Extremely Loud & Incredibly Close. It's unconventional in many regards and techniques, and I am especially interested in the slow-moving character developments. The protagonist, Oskar Schell, is who we are given the most detailed information about. The other characters are introduced in regards to him and their relationship, such as his mother, father, grandma, and people he has come across thus far. But then there are chapters that are separated from Oskar's story. Foer does not explicitly say who the person is that is narrating these chapters, but I am guessing through context clues and hints that they are focused on Oskar's grandparent's story. In the Why I'm Not Where You Are 5/21/63 chapters, the narrator (Oskar's grandfather) is writing a long letter to his unborn child (Oskar's father). These chapters are my favorite so far. I like the mystery in them, the confusion they create, the anticipation of knowing everything will somehow connect later in the novel.