A Scanner Darkly by Philip K. Dick
"A Scanner Darkly" was published in 1977, so the fact it is set in the 1990's means it was "the future". There is a fictional illegal drug in use, and there are a small number of futuristic devices used (for instance, police surveillance using holographic recorders). But generally, the book pictures a somewhat extrapolated view of the 1970's California drug scene. Not so much the tamer Cheech And Chong world in which they use only comparatively harmless drugs to humorous excess. The main characters here use a highly-addictive drug that tends to cause serious brain changes / damage in less time than done by drugs familiar to us. But the drug culture life style is familiar.
The story revolves around "Fred", a narc infiltrating the local drug scene - seeking to get contacts to follow up the ladder to arrest some higher-up figures. He's living the life of a counter-culture drug user. Including using "Substance D" - the drug mentioned above - which is also nicknamed "Sudden Death" by its users. The way these narcs work is they have one identity when doing their undercover work. When they meet with other police to discuss matters they wear a "scramble suit" that disguises their appearance and voice, and they use a different name. This way criminals can't get information from crooked cops about who the narcs are.
Fred's been trying to get evidence on one suspect - who then disappears. His superiors then reassign him to investigate another person in the drug scene. But the person they now have Fred watching is his other drug scene identity. He must play along to maintain the secrecy of his identity. Holographic surveillance equipment is put in the house he shares with two other users. And strange things start to happen. Not long afterwards, his use of Substance D begins to impair his brain so his two identities become a split personality.
That's the framework. Much of the bulk of book is thought sequences going on in drug-influenced minds or dialogues of people heavily under the influence. There are people constructing possible scenarios in their heads. There are groups of people getting into in-depth contemplations on subjects comparable to "Why do spaghetti noodles get into such twisted arrangements?" or debating over something like "If vegetables were intelligent what kind of music would they like?" Dick's presentation of these things works reasonably well for me, even if the above description sounds unappealing. Perhaps this reflects the fact just saying the topic makes it sound worse than it is. Nevertheless, I don't assume this will agree with everyone's tastes.
Other parts of the book have to do with the complex interactions between big-time drug dealers and police narcotics work. Parts have to do with the damage done by the drugs to various users.
Dick closes the book with an afterword about the drug scene, the damage to users and references to his own involvement. He makes a point that he is not trying to say that drug users did bad things and got what they deserved if they died. Rather, users in the drug scene may have been too much into “play” and not enough into responsibility – but the “punishment” was too severe for this shortcoming.
Especially looking at a book that supposedly takes place in the 1990's from the vantage point of the 2000's, it might make a bit more sense to think of A Scanner Darkly as something like "alternative history" than what is more often referred to as "science fiction".