Who is Grandfather from A Ring of Endless Light and what is their importance?

The Austin family comes to the island for a few weeks each summer. This year, they are there for an undetermined time frame because Grandfather is dying of leukemia. They're there to help make his life comfortable. He is a quiet, gentle man who cares more about the comfort and reactions of others than he does for his own comfort. He has an intense love of literature and a close relationship with his children and grandchildren. He enjoys sharing literature with the family, even uses it to solve disputes. He is a mentor for Vicky and encourages her in her writing. Grandfather has a steady belief in God and heaven, and he is ready to embrace death. He was a preacher who also spent part of his life writing down the history of a remote tribe in Africa. He begins to lose his grip on reality as the book progresses, and the disease takes over his body.