Once In a Very Blue Moon - Nanci Griffith (The link is a video snippet of the song.)
Same thing for the Mekons’ “Love Letter” - the vocalist starts by singing about the man writing a letter to his ex, and sings some of the lines, but it diverts after the first verse.
Chelsea Hotel #2 also by Leonard Cohen, is a letter of sorts to his dead lover Janis Joplin.
Thanks for the suggestions! A lot of these I had heard before and liked. It’s a kind of cool genre, time to make a new playlist 
Would a Prayer to God count?
Santa Baby
And I wrote this letter in my head…
Dear Prudence ~ The Beatles
Andrew Lloyd Webber’s Song & Dance has First Letter Home, Second Letter Home, and Third Letter Home.
Bob Schneider’s 2002. So melancholy and self pitying, it can’t help but be one of my favorite songs.
“Strawberry Letter 23” , the best known version of which was by The Brothers Johnson.
Incidentally, before I opened this thread I thought it was going to be about things like:
Van der Graaf Generator: W
Jackson Five: ABC
Cream: NSU and SWLABR
Rush: YYZ
"LBJ took the IRT
down to 4th street USA
When he got there, what did he see?
The youth of America on LSD.
LBJ, IRT
USA, LSD
LSD, LBJ
FBI, CIA
F-B-I-C-I-A
L-S-D;
Eh-eh-el…Be…Jay"
Hmm…not to sure about that one. I always pictured some guy standing in her front yard yelling up at her window “Hey, Prudence, come on out and play”
“Signed D.C.” by Love ( Arthur Lee )
“The Letters” by King Crimson (though it’s more about a sad correspondence than the actual words in the letters
“Fucker” by Eels
“Dear Darling” by Dennis DeYoung
This one is obscure, but well worth hunting for.
Dear Darcie by
Steven Fromholz from his “A Legend in My Own Mind” album.
Note: The lyric site has it as “Darcy,” but the cut on the actual album is “Darcie.”
In fact, if you run across any of his stuff, snag it. You will not be disappointed.
**Paul Kelly’s ** [How to Make Gravy](Designer forms are structured documents with a hierarchical structure that can be converted into XML. This structure can include structure from XML Schema and example XML files. ). But don’t listen to it when you’re emotional.
Nas - “One Love”
The Bloodhound Gang - “The Ballad of Chasey Lain”
How 'bout some old-school country music:
Tupelo County Jail - Webb Pierce