"Notes Toward A Novel of Love in the Dog Park," by Louis Bayard, in Ellery Queen's Mystery Magazine, November 2011.
But this story isn't exactly made up of letters. As the title suggests it is notes for a novel, complete with earnest quotations from some writing manual. The question is how much of the scary plotting of the book's narrator is actually things the would'be novelist has done, or is planning to do.
Use passages from journal here. E.g. stripping skin from Ellen's face.
I'm still not sure of exactly what is real and unreal here, but it is a delightfully disturbing tale.