Remarks on the Doomsday Argument  
   Questions if people are underestimating the likelihood of human extinction in the next few decades, a position put forward by the philosophical Carter-Leslie doomsday argument. Highly philosophical and statistical.
   http://www.anthropic-principle.com/preprints/inv/investigations.html

Book: The End of the World - The Science and Ethics of Human Extinction  
   by Prof. John Leslie. The first comprehensive survey of potential catastrophes speculated upon by scientists, from comet and asteroid impacts to nuclear or chemical warfare, from ozone layer destruction to overpopulation and poisoning of the environment o
   http://www.amazon.com/exec/obidos/ASIN/0415184479

Existential Risks  
   Paper by Nick Bostrom, analyzing well-known and less well-known extinction risks and related crippling hazards to humanity.
   http://www.nickbostrom.com/existential/risks.html

Beyond the Holocaust - Survival or Extinction?  
   Subtitled "A survival manual for humanity", this online book attempts to summarise problems threatening humanity's very survival, their causes and possible solutions to them.
   http://www.hpn.org/beyond/