You know the one with Roland's name in it.

After Eddie saves Calvin's bacon from Balazar's men they are having some good coffee, they are talking about book and Calvin comes up with this letter (from his great-great-grandfather) he has always kept in a safe and there is a damn old piece of paper in it with Roland's name on it.
So...
how could Calvin's great-great-granddad know about Roland? Why did he write such a letter? Did he know that his son would be visited by gunslingers once?
How?
How?
How
And again how?