I wish I could point you in the right direction but I can't tell you any more than google can. I just assume it's not a random pull from the pile.
I just like the fact that these letters don't end at his eyes. He passes them along to the people who are going to draft the policies he plans to put into action. That is what I like about this whole thing and it means more to me than the fact that he reads them in the first place.