I wrote a seduous fan letter to Mr. Stephen Sondheim in the early 80's with a few specific questions about songs and shows, which he kindly answered promptly, efficiently, and with great style.
In the mid-80's, I wrote letters to Ned Rorem and Lukas Foss, both of which were answered most beguilingly.
I also wrote a letter to Mrs. Coretta Scott King when I was in grade school; sadly it was never answered.