Through A Scanner Darkly is a Phillip K. Dick book- one of the ones I looked at when I heard that Paycheck was just a short story (which I kind of wanted to read as a short story- A Scanner Darkly was another story in the book)
It's the story about a cop who goes undercover to do a drug sting- but in a future where mind readers are common, there's no real way to go undercover- so police create alternate personalities, ones that can be taken on and off at will, so they can hunt criminals anyways.
I think it's the story of a cop who is hunting this huge drug dealer, who keeps outsmarting him somehow.
I'm not going to give you any spoilers, but I really want to see it.
-kv