Hi!
Similarly the "Diary of Anne Frank" is an instance of a memoir or a literary work but is a subclass of book (because there are lots of physical books with that name). Literary works have authors and publishers. Books have numbers of pages and printers and physical locations.
I'm not sure I understand this. What is the difference between "instance of memoir" and "subclass of book"? You could literally argue with the same words that it is also "subclass of memoir" and again since very rarely any specific physical book is notable enough (maybe excluding things like first Gutenberg Bible, etc.) we would have virtually no instances of book at all. I do not think people think that way - if you ask somebody is "Diary of Anne Frank" an example of a book or a class I think most people would say it's an example of a book and not a class. Unless we plan to seek out and record every printed physical copy of that book, I don't see any practical reason to describe it as a class. This class - and hundreds of thousands of other book titles, maybe with rare exceptions of the Gutenberg Bible, etc. - would never have any instances. So my question is - what is the use of modeling something as a class if there won't be ever any instances of the class modeled?