Philip K. Dick's A Scanner Darkly is set in a future dystopia (1994), but it's very different from other dystopic literature I've read: instead of being concerned with a sweeping new vision of a future society, it's concerned with smaller, more personal issues. In it, a government agent named Fred is assigned to infiltrate the heirarchy of the drug world, and in his disguise as run-of-the-mill druggie Bob Arctor he becomes addicted to a mysterious drug known as Substance D.
The thing about Substance D, however, is that it dissolves the corpus callosum, the bridge between the two hemispheres of your brain and can result in the development of two different personas. Because not even Fred's superior's are allowed to know his secret identity as Bob Arctor, Fred is assigned to spy on himself. At first what is a mildly comic result of Dick's police state becomes a bizarre scenario in which Fred and Bob separate, and neither realizes that they're the same person. The title is a reference to the Bible, which refers to seeing oneself "through a mirror darkly," as Fred/Bob surveys his own life through a "scanner," which is basically a glorified surveillance camera, unable to look upon himself completely.
Supposedly, writing this book was agonizing for Dick, who based much of it upon his own experiences as a drug addict. The dialogue in the book between Bob and his drug-buddies is both surreal and sincere, reflecting a genuine but skewed view on reality that wavers between comedy and tragedy. The destruction of Fred/Bob's mind is heartbreaking, and given extra force by the epilogue, in which Dick dedicates the book to all of his friends who died or were permanently damaged by drug use--including himself, listed with the rest, as having severe pancreatic damage due to his addictions.
This novel is probably the most stirring rejection of drug use that I've ever read. Somewhat peculiarly, the Times 100 Greatest Books of the 20th Century list only includes Ubik from Dick's extensive corpus, instead of one of his more popular novels like this one, The Man in the High Castle, or Do Androids Dream of Electric Sheep?, but I can't imagine any of his books being more genuine or affecting than this one.