The 7/2005 letter from al-Zawahiri (Osama bin Laden's #2 in Al Qaeda), to Zarqawi, the head of al Qaeda in Iraq. Amongst the religious rhetoric, it lays out goals, strategy, and suggestions for PR (like "slaughtering hostages in public is probably not going to bring the public to your side").

Interesting reading.

Letter from Zawahari to Zarqawi