Having just finished The Death of Grass ( http://en.wikipedia.org/wiki/The_Death_Of_Grass ) I've got to says it's taken the place of my favourite, and most scarily plausible collapse of society I've ever read.

Only of course nowadays things would likely be more violent, less pitying and far fewer people would have the skills to survive than in the post-war setting of the novel.

other previous favourites: Fahrenheit 451, We, 1984 etc