Hint 1
Hint 2
Hint 3
Hint 4
The letter from Mitchell says he works for The New Yorker magazine. Look up "New Yorker" in the phone book, then call and ask for Mitchell. That location is now on your map.
If you've already talked to him about the musical ghost --
The Musician
-- you'll be pleased to find that he's a lot more forthcoming about Mavis than he was about Isaac.
Come talk to Mitchell again after you talk to Mavis's son to find out what kind of gift he gave his mom.
How do I find Mavis's son?