Mark Evanier trots out the tired and fallacious old defense of bad movie adaptations, "the book is still on the shelf."
Here's the letter I sent in reply. I got no answer, either by e-mail or in a blog post, but then I never do from him. (The Three Stooges context was his reference.)
I have to raise objection to your anecdote about the author (I hear the story often, and usually it's said to be James M. Cain) who said, "My book isn't ruined, it's still right there on the shelf."
That may be true when the book, like Three Stooges movies, has no dignity. But when it does, no matter how beloved and oft-read the book is, that dignity can have its hair pulled or a tire iron smashed over its head, and, unlike when Moe does it to Larry, it hurts.
A book sitting on the shelf isn't alive, it's dead. It doesn't live unless somebody takes it down from the shelf and reads it. When they do, it comes alive in their imagination. That's the thing about books: unlike movies which tell you exactly what everything looks and sounds like, books leave a lot to your imagination. And if your imagination has been hijacked by a movie of that book, your individual creative imaginative experience has been ruined.
It will happen. Movies are powerful experiences, and while they may sometimes be forgotten, they can also make a permanent impression. You can see this in little things. Fan drawings, in science fiction convention art shows, of characters from The Lord of the Rings used to show a variety of imaginative interpretations. Now they all look like the actors from the Peter Jackson movies.
Second thing: A movie can take over the conversation about the book, even when the conversation is specifically supposed to be about the book. I've seen scholarly articles about Tolkien which say things that are true of the movies, but not of the books the article is supposed to be about. And the general run of readers today know nothing about Frankenstein or The Wonderful Wizard of Oz, even though they're still much-read books. People are always completely surprised when they pick up the books and find out how unlike the famous movies they are. The movies have drowned the books out.
Lastly, sometimes it's not even true that the book is still on the shelf, if that shelf is a bookstore shelf. Sometimes it's been replaced by a novelization of the movie.