'a scanner darkly' what an awful reading experience. Film also dogshit.
What? Really? I was totally blown away by the book? I thought it was genius the way it captured the paranoia arising from the police infiltrating the gangs and the gangs infiltrating the police. The way that the undercover cops have to disguise themselves when they are back amongst the police so that a spy can't reveal their identity - and then when this is combined with the dissociative effects of substance d and the descent into madness that is arising from its abuse so that you have the mole not knowing which person he is and whether or not the person he is spying on is himself or not. When that happened I found that a genuinely vertiginous idea that properly fucked with my head.
And it's also a very complex idea that he manages to make hit you forcefully in a way that wouldn't work if it wasn't done with consummate skill. It has to be built up to and delivered in such a way that it knocks you off your feet as much as it does the protagonist. It has be to be explained well enough that you get it but not so didactically that it's dry.
So I thought that that book, in particular, succeeds extremely well in fucking you up and scaring you with ideas that had to be elegantly constructed and handled or they would have fallen flat - and taken the whole book down with them. In short I would make A Scanner Darkly (also a good, attention grabbing and intriguing, title, though some might argue that it's a tad clumsy) exhibit A in terms of evidence for his being a good writer.
I'm also surprised you don't like the film. It is necessarily slightly simplified compared to the book, but for me it was one of the more complex sci-fi films of the era that does take on most of what is contained in the book while at the same time being a lot of fun and rattling along at a decent pace, always watchable and with that cool effect. I think it's great.