It's actually surprisingly difficult to determine what constitutes a "famous" work. Is it the number of sales? Is it its later influence? Is it how many people have heard of it? What about a book like, for example, The City of God, where the number of people who have heard of the book far exceeds the number of people that have actually read it? How do you even determine how many people have heard of a book? Are we going to send out a survey every time we want to post a question?
It can be extremely difficult to tell how many people have heard of a book, or what the direct or indirect influence of a book has been. For many older books, it can be difficult or impossible to determine how many people have read it or even what total sales have been. For example, how many people in ancient Greece read Aristotle's Nichomachean Ethics? What were its total sales? How many people read it between 1600 and 1610? Obviously, most people would acknowledge that as a famous and influential book, but the point is even for such a well-known book it's simply not possible to quantify exactly how well-known or influential it is.
There are so many edge cases, and so many cases where it's difficult or impossible to tell how "famous" a book is, that it would be impossible to set any kind of general standard for this.
If someone posts a question about a book that you feel isn't worth considering, you are, of course, free to downvote the question, but we really can't set a general standard for a book not being well-known enough to be on-topic.