I wonder if [Alan] Moore is "big" enough that someone will come along and do Moore "right" without actually, specifically adapting one his works. I'm thinking of the way Eternal Sunshine of the Spotless Mind captured the low-rent sci-fi feel of Philip K. Dick's stories better than any of the official Dick adaptations.
So, we're talking about movies that aren't actual adaptations that "get" something about a writer better than any other movie, especially movies that are adapted directly from the writer's work.
The Eternal Sunshine of the Spotless mind is my go-to example: it nails the next-week-in-Hoboken feel of Philip K. Dick's stories better than the big-budget, effects-packed movies that are actually adapted from them.
Another example: The Third Man, where Graham Greene and Carol Reed do a better job of bringing the sensibility of Eric Ambler's spy novels to the screen than the movies taken directly from Ambler's novels (even the ones where Ambler worked on the screenplays).
Do any other movies fit?
If I recall correctly, Pauline Kael made this kind of point about Robert Altman's Thieves Like Us and William Faulkner.
Some more of my own suggestions: 28 Days Later - J.G. Ballard's sci-fi novels, Fingers and Black & White - various pieces (essays, novels, etc.) by Norman Mailer.
Anyone else have any?
(As a starting place, here are a couple of authors who, I feel, haven't been done justice by the movies based on their work: Joseph Conrad, Jack Kirby, Donald E. Westlake. Elmore Leonard would have made this list if it weren't for Out of Sight.)