There are so many. These are the ones I could think of off the top of my head:
Pretty much the entire musical Daddy Long Legs.
Desiree's sections of "The Glamorous Life" from A Little Night Music
"Dear One" from Kiss of the Spider Woman
"Here Alone" from Little Women
"At the Glen" from Dessa Rose
"Eleanor's Letter" from First Lady Suite
"Letter to the Governor" from Parade
"She Used to Be Mine" from Waitress starts out as Jenna writing a letter to her baby.
"Edges of the World" from Fun Home
"A Summer in Ohio" from The Last Five Years
"And Then There Were None" from Spring Awakening...kind of (the song is framed around Frau Gabor writing Moritz a letter, to which Moritz responds in song)
"The Proposal/The Night Was Alive" from Titanic is set around sending a telegram
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body
