Tim Parks in the NYRB suggests that finishing a book may not be necessary to the aesthetic experience. I don’t agree with this line of thinking, that the concept of the art in question lies entirely with the reader or viewer. If a piece of art (i.e. a book or painting) is conceived as a whole and executed as a whole, then finishing it is necessary to understanding and appreciating that piece of art.
If you are not enjoying a book, feel free to put it down. I have many times. But I don’t pretend that I have formed a complete and valid judgment. I forfeit that when I fail to comprehend the work as a whole.
As for whether endings are “necessary” when you have enjoyed a book, it depends on how necessary the author intended it to be, not on whether (as the author was rightly, in my opinion, angered by) the reader felt they were “done.” It’s a bit like knocking the wings off a statue because you think it looks better that way. What you think looks better isn’t the point. The statue was created that way because that’s the way the creator conceived it.
Personally, I think it is critical to read every word as the author intended. Otherwise you are appointing yourself as editor over their artistic imagination. You are in charge of your own time and enjoyment, but not the structure and content of their work.