Informative in a friendly format, the 9/11 Report is a graphic adaptation of the official 9/11 Report issued by the 9/11 Commission.
It offers the history of the intel and preparedness before the terrorist attacks, the response during and in the immediate aftermath of the attacks, and subsequent actions up to the date of publication.
The Commission offers suggestions for changes and finds fault at every aspect of communication and preparedness. Hardly surprising.
I found it easy to follow and read, but not condescending. If you don't have the time or inclination to read the full report, this will offer a respectable synopsis.