I read this website a while ago which was really interesting to me: https://letterlarp.home.blog/why-is-it-called-letter-larp/
Otherwise I think best known letter game is De Profundis - https://www.rpg.net/reviews/archive/classic/rev_6859.phtml
I published one to my patrons this month but it isn't on general release yet - sorry that isn't super helpful.