It's not exactly a letter, more like the older man wrote an entire book about his feelings for the younger and published it for the world to read..the book has a sad ending but the older s love doesn't (✿^‿^)
You may feel your favorite manga should be gathered together into distinct categories for your own reference and, now, you can do this with a LIST. After you've created your list or lists, you can proudly recommend them to other manga fans to showcase and share your taste in manga.
Samishigariya no Love Letter