Ah, another fun one. Here are the rules:
Read at least four books about "the end of the world." This includes both apocalyptic fiction and post-apocalyptic fiction. There is quite a bit of overlap with dystopic fiction as well. The point being something--be it coming from within or without, natural or unnatural--has changed civilization, society, humanity to such a degree that it radically differs from "life as we now know it." (Aliens, evil governments, war, plague, natural disasters like earthquakes, tsunamis, volcanoes, hurricanes, depletion of resources, genetic manipulation, etc.) Here is a wikipedia article on the subject. Also see here. These changes can be small-but-still-significant or huge-and-life-threatening.
The Road by Cormack McCarthy
Life As We Knew It by Susan Beth Pfeffer
Liberation by Brian Francis Slattery
A Canticle for Leibowitz by Walter M. Miller