I have always loved this book. The extended comic riffs - like the discussion about the microdot factory and the argument over the bicycle gears - are positively cackle-inducing, and yet the inevitable slow disintegration of the main character is as dark and bleak as all hell. And at the end, perhaps, a faint glimmer of hope, but not for anybody we've seen in the book. To underline that point, it ends with a factual list of the drug-damaged friends of the author, and the book is dedicated to them.
It's also one of the few books where I can recommend the movie version, by Richard Linklater, which is artistically crazy (but it works) and features Robert Downey Jr. in excellent form as the sinister narcissist Barris.
The Quietus reports on the 40th anniversary of A Scanner Darkly : Collapsed Horizon: Philip K Dick's A Scanner Darkly, 40 Years On by Eli Lee , January 15th