In the end, Jacob figures out who his grandfather really was, and is still undecided about whether he wants to let his grandma in on the big news. And then the book ends. We're not sure whether he ends up telling his grandma or not, but we know it doesn't matter to the book.
How can we say that? Think about it. The novel isn't about his grandma at all—it's about Jacob and his journey. So in the end, it doesn't really matter if he tells her or not, because the whole novel was about him finding out more about his grandpa, his heritage, and himself. We think the author purposely left it up in the air so we have to decide what happens next. Besides, Shmoopsters, it's all about the journey in the end—and what a long, strange trip it's been.