A Scanner Darkly is a brain-bending, drug-addled quest for identity. In other words, it feels much more typical of Philip K. Dick than The Man in the High Castle.
Bob Arctor lives in a household of addicts to a psychoactive drug called Substance D, but he is also Agent Fred, an undercover cop spying on Bob Arctor's household of bumbling drug addicts. It's basically the old thriller plot - "agent gets lost in deep cover" - with a humorous and satirical twist. Bob and his friends are incredibly paranoid, and Bob/Fred is so far gone that he doesn't even realize that he has very good reasons to feel that way. The two halves of his consciousness work independently against each other for most of the first half of the novel, while his listless household goes through the normal dysfunctions and tribulations of the perennially stoned. The latter parts of the novel are grimmer, as they follow a damaged Bob/Fred through his time in a rehabilitative work camp.
Dick - as anyone who has read a few of his novels is sure to notice - was an addict himself, so he's speaking from experience here. He never glamorizes drug use (in fact, the novel can be quite chilling at the same time it is darkly comic), but the novel works best as a commentary on the drug war (which grew throughout the '70s and came to dominate the American legal system by the '80s). It begins to feel like all of the drug users and dealers are undercover cops, destroying their own and each others lives while doing little to help the real addicts. It's some of Dick's finest work. It's also perhaps his most accessible work, even while it still deals with his core themes of identity and reality. This is probably the best entry point to Dick other than perhaps Do Androids Dream of Electric Sheep? I'd also recommend the Richard Linklater movie as the best PKD adaptation (not to say it's better than Blade Runner, just a better adaptation).