Michael Billington at the Guardian asks "Have any unarguably great novels actually been improved by adaptation?" He is referring to adaptation to cinema or the theatre. This kind of question is asked all the time, and is one of the questions that comes up whenever some great (or simply popular) book comes to the screen or stage. How many times have you heard someone, perhaps yourself, say, "I don't think the movie was as good as the book"?
But if Billington's question is part of a discussion about whether a novel should be dramatized, then it's the wrong question to ask. Just because a story as it is portrayed in a movie is not as good as it is portrayed in print is irrellevant to the quality of the movie or whether it should have been dramatized in the first place.
The question is not whether a novel is improved by its dramatization, but whether dramatization is improved by having been based on a novel. What is important is not whether a novel is "improved by adaptation" (how could a novel be improved by anything other than the author rewriting it?), but whether a movie (and the same goes for a play play) is a better play for having been based on the story from a novel--as opposed, say, to having been written from scratch by a screen writer or playwright.
The question is not, for example, would I rather go and see the movie "Sense and Sensibility" or read Jane Austen's novel. I can do both and profit by it. The question is whether I would rather go see the movie "Sense and Sensibility" or some other movie based on a story by what is likely to be an inferior writer.
The comparisons between movies and books is seldom helpful. They are very different art forms, and should be judged by very different criteria. The purpose of a movie is not to be a good book, any more than the purpose of an apple is to be a good orange. Nor is the purpose of a movie to improve a book through adaptation.
The purpose of a good movie is to be a good movie, and that purpose is often served by adapting the story from a good book.