Down In Flames
Down in Flames is about the end of the world. It provides ideas on how to drastically (but coherently) change your setting based upon several forms of major disaster: natural disasters; man-made disasters; 'outside-influenced' disasters; and wrath of (a) god disasters. In each category the book cites three possible scenarios. And it tacks on a final scenario to bring the total to an even baker's dozen of world-cracking fun.