I nominate Roadside Picnic
by the Strugatskys brothers. The film Stalker
is loosely based on the novel.
Here is the plot background from Wikipedia:
The novel is set in a post-visitation world where there are now six Zones known on Earth (each zone is approximately five square miles/kilometers in size) which are still full of unexplained phenomena and where strange happenings have briefly occurred, assumed to have been visitations by aliens. World governments and the UN try to keep tight control over them to prevent leakage of artifacts from the Zones, fearful of unforeseen consequences. A subculture of stalkers, thieves going into the Zones to get the artifacts, evolves around the Zones.
From what I know the book has no copyright and can be found online for free.