There is also a short story that makes a huge twist on the story from the Picnic. PS. The antology that contained the story was called Время учеников. I doubt it was translated to English though.

Roadside Picnic was so depressing though. Have you red the sequel? It made it even more depressing I think. (it was a short-story written by someone else but backed by one of the original writers, it was done to promote young writers in Russia I think)