If you want to read more into what we knew about the role that sectarianism could potentially play, here are three posts with links to other sources
OIF Research Library - Reconstructing Iraq authored out of the Army War College
OIF Research Library - Future of Iraq Project headed by Warwick from State, and promptly excluded by SecDef Rumsfeld from participating in planning despite being sent by SecState Powell to assist
Iraq in Books - Review Essay - A potpourri of books on Iraq, but the specific book that I would recommend as the very first book for anyone that wants to take a serious look Iraq would be the first book listed by Ahmed Hashim. If you want a shorter version, you can go to some articles that the Jamestown Foundation has:
Ahmed Hashim