A very important book on Al Qaeda and OBL. Bergen gives a very detailed history of the relationship among the Shia, Sunni, and the Kurds. One of his points was that, in Hussein's Iraq many communities were "integrated," Shia and Sunni families sharing the same neighborhood. He emphasized the results of US'incompetence in starting the war without understanding the ethnic/religious history and contemporary situation; allowing looting of weapons caches and labs capable of making WMD without security; and no real plan for what to do after the war overcame the Iraqi leadership.
I've read lots of this by other writers and in articles by Peter Bergen but his version is much more contextualized. Besides containing so much information, Bergen's book is very readable.