Soufan was an FBI agent who specialised in Al-Qaeda long before 9/11, in fact even before the attack on the USS Cole. His experience as an interrogator revealed significant findings of their organisational structure and plans, which helped to break up a number of terrorist plots.
Soufan showed through his interrogations that Enhanced Interrogation Techniques, such as waterboarding was entirely the wrong way to interrogate people, using knowledge gained from previous interrogatted people was the best way to find out further intelligence.
It appears the CIA chose to undermine all the good work carried out and proceeded to use EIT's in Guantanimo Bay. Using EIT's enabled the CIA to find a link between Iraq and Al Qaeda that didn't actually exist. Preferring to use EIT's meant killing Bin Laden took years longer than if the FBI interrogators could have interviewed various suspects.
The CIA does not come across well in this book, from not sharing information pre 9/11 with the FBI to having large sections of this book Redacted (both the FBI and US state dept give the book the OK in its original format). Some of the Redacted words are ridiculous, can easily be worked out what is missing, but large chunks of text are also missing making sections harder to read.
But this is still a fascinating insight into Al Qaeda and highly recommended for anyone interested in 9/11 attacks and Al Qaeda.