Malcolm Nance has updated his 2007 book, which IMHO was essential reading then and he emphasised the part played by former regime loyalists (FRL) in the insurgency. It was published in December 2014 and has two reviews:

The author, a SWC member and rare poster is:
a counterterrorism and intelligence consultant for the US Special Operations, Homeland Security, and Intelligence agencies, with more than 33 years of experience in combatting radical extremism. An honorably retired Arabic-speaking US Navy intelligence collections operator, field interrogator, and survival, evasion, resistance, and escape specialist, he spent two decades on clandestine antiterrorism and counterterrorism intelligence operations in the Middle East, North Africa, the Balkans, South Asia, and sub-Saharan Africa in support of the Special Operations and Intelligence Community. He is currently executive director of the Terror Asymmetrics Project on Strategy, Tactics, and Radical Ideologies (TAPSTRI), Hudson, New York, USA.
Link to TAPSTRI's website:

There is an old, closed thread that his book prompted Al Qaeda in Iraq, which is worth a skim and ended in 2011: