I wasn't familiar with this book, but I had at least heard of the movie and video game Stalker, which apparently are at least loosely based on it. It's a work of science fiction written by Soviets, but, as the introduction notes, not really a work of Soviet science fiction: it's hard to discern much ideology here, let alone one upholding the Soviet worldview.
The concept behind the book is pretty fun. It's a first contact story, but not the stereotypical form, where humans and aliens meet one another and exchange words or gunfire. Instead, a series of impacts strike Earth in five (I think) places, leaving behind a bizarre wasteland. Roadside Picnic focuses on one of these areas, called "The Zone". As with much science fiction, part of it focuses on the contents of The Zone and the technology or xenoanthropology it may reveal, but it's more interested in the human response to the incident: how it feels to live next door to a warped location, the legal and underground economies that spring up to exploit it, the academic controversies and prestige and popular interest, governments attempting to maintain stability in the face of an existential threat, and so on.
A few characters in the book are scientists who explain what they know and don't know about the Zone and its contents, but most of the characters are just ordinary folks who happened to be living nearby and have adjusted to life there. The main character is probably Red, who is a "stalker": someone who illegally enters the Zone to find and retrieve alien artifacts. This is a life-threatening operation: there are "hot zones" that suddenly flare up hundreds of degrees, carnivorous plants, "hellslime" that decomposes anything it touches, and other bizarre and unpredictable threats. The loot is fascinating but also deeply odd; batteries that never run out of power, marbles that hover in the air, and more. And we eventually learn about still more strange effects of the Zone, in particular how previously deceased individuals have come back to life and shuffled back to their old homes, somewhat zombified but still carrying an echo of what they were in life.
The title "Roadside Picnic" is eventually described in a late conversation between a harried government bureaucrat and an inebriated scientist. The scientist makes an analogy: imagine that you went on a weekend holiday with some friends. You rented a car, drove out into the country, found a good spot, laid down your blanket, ate lunch with wine and cheese and bread, maybe played some croquet, then got in your car and drove back to the city. As soon as you leave, the ants come crawling back. They see the grass you trampled, the crumbs of delicious food, an errant croquet hoop, rubber tracks on the edge of the road. All of this is vastly outside of their experience and comprehension, and now is an integral and important part of their world. They might wonder what the visitors wanted with them, whether they are being punished, what they should do to prepare for a return visit. The reality is that the visitors didn't spend an instant thinking about the ants, don't care about them at all one way or another. In the scientist's opinion, this is the exact situation that the Earth has found itself in. This is an "encounter" with an alien species, but in the same way that a baseball encounters a pane of glass, not like two humans encountering one another in a cafe. (And, as I write this out, I realize that this thesis somewhat resembles The Dark Forest theory in Cixin Liu's Three Body Problem.)
This was a good read, and I'm looking forward to checking out the Tarkovsky movie; from what I've heard it isn't a super-faithful adaptation, which is fine by me, it gives me something new to experience!