I'm also going to go with Zoe Heller's What Was She Thinking?: Notes on a Scandal and Ron Hansen's The Assassination of Jesse James by the Coward Robert Ford. Both are excellent novels; in the former Heller's deliciously acidic observational prose glimmers like a black diamond and the later is just a brilliantly moody and lyrical piece of southern gothic writing.
But the film adaptations are even better. In Scandal Judi Dench has such presence as our calculating, judgmental old harpy at the heart of the story, masterfully playing the emotional mosquito to Cate Blanchett's ethically jumbled and conflicted girl-woman teacher.
And in the case of Assassination....Jesse James, it's just one of my favorite American films of all time. The pacing, mood, lighting, acting, casting choices, locations, psychological undertones....Andrew Dominik learned his lessons well from Terrance Mallick on this one (too bad Killing Me Softly was such crap though). Even the extensive V.O. narration, often a hackneyed and obvious device in most films, is stellar here, doubtless because large swatches of dialog are lifted verbatim from the source novel and the poetry in the prose is evident.
Good examples of the potential of film as an original adaptive force, to retain the spirit of the source material whilst remaking it through its particular audio-visual lens.