Not entirely sure what part of my message this is aimed at.
If you have the physical book in the rerelease form then there is a short story connected to 耳をすませば, not sure if it is a sequel or what exactly. Many manga/books have 後書/あとがき where the author talks about whatever they feel like, usually something in connection to the story you just read. There is also an essay and an interview article in this volume. Those last two I mentioned aren’t standard, but you find such things in some things.
Yeah, the essay-by-some-other-person (the 解説 – かいせつ) is rarer for manga, but it’s extremely common for (non-light-novel) novels – most will have one IME. They’re generally 5 to 10 pages, and the writer will say something about what they like about the book or how great the author is or mention some personal connection to the work. They often have a bit of description of the book – I’ve read several that claim that some people who pick up books in a bookshop will first turn to the 解説 to see whether they’ll like the book (I guess the equivalent of reading an Amazon review :-)). They are of somewhat variable quality and interest, but I always read them because I’m a completionist about reading books.
In this case the 解説 is by 鈴木 敏夫 (Suzuki Toshio), the producer of the Ghibli anime adaptation, and the discussion is between the manga author and 近藤 喜文 (Kondō Yoshifumi) who was the director of the anime (and who sadly died rather young).
I will be reading the second story, 解説, discussion, etc, from the physical re-release version, so if anybody else does and has questions about them I’m happy to try to help if I can.