"Roadside Piknick" (often called "Stalker") is a very popular book by 2 of Russia's best Philosophical Sci-Fi writers, the Strugatskiy brothers. They wrote 40 years ago.
It's basically about alien debris on earth (hence the radioactive Zone) and people (stalkers) who make profit by gathering and selling them.
The game is BASED on those books, the developperss said that it's too difficult to make an action game out of a philosophical sci-fi, so they modified it.
Look on google if you wish, you might find the books in english. I remember I posted the link a while ago in the RPG Forums here, a topic about S.T.A.L.K.E.R.
