Recommended (in order):
1. Eaarth: Making A Life On A Tough New Planet by Bill McKibben (2010). Lucid and affecting, it drives home how we have changed our planet, and what that means for the future. An excellent introduction to the subject.
2. Heat: How To Stop The Planet From Burning by George Monbiot (2007). A rigorous and unsparing assessment of how major cutbacks in greenhouse gas emissions could and must be made. [my mini review] 3. Six Degrees: Our Future On A Hotter Planet by Mark Lynas (2008). A degree-by-degree breakdown of climate effects from one to six degrees Celsius of global warming, with a global scope. Fascinating and fearsome. [my review]