This just in from Jim Korkis:

[One of my joys when I was younger was visiting the "Walt Disney Story" at Disneyland.  As part of the pre-show exhibit was a letter from silent screen actress Mary Pickford who was a huge Mickey Mouse fan.  I was fascinated.  The letter is currently being displayed at a new D23 exhibit in Chicago.  I thought that there may be other people besides myself that would love to see a copy of that 1930 letter.  A few years later United Artists (run by Pickford) would distribute the Mickey Mouse shorts.  I wish I had thought to include it in The Book of Mouse but I just didn't have room for anything more.]