Barack Obama's mother is a pivotal figure in his life, as she raises him much of the time, pushes him to seriously pursue his education, and is one of his inspirations. That being said, her failed marriage to his father and her remarriage to an Indonesian man who relocated the family are the source of some tension for Obama.
His Grandmother was his surrogate mother in the time when his own mother could not be there in Hawaii to raise him. She is compassionate and loving towards him, and provides a stable home for him during his adolescence.
His Grandfather becomes his father, essentially, and Barack Obama establishes a very strong bond with him as the primary male role model in his life. He only sees his actual father on a couple of occasions, so his grandfather plays a central role in his upbringing.
Obama makes very clear in this book how important these three are in his life. He speaks of them in glowing terms through most of the book, with only occasional exceptions.