I linked this on my StumbleUpon site, but I may as well put it up here too, as it shows some pretty hard and conclusive data concerning Al-Qaeda. The article explains it all really, and while I don't yet have the book (too busy buying Jared Diamond stuff), it is most certainly considered brilliant by many others and well worth owning. As the article explains, Sageman is an ex-CIA psychiatrist who was involved in Afghanistan the first time around, when the Soviets were there. He also managed to interview or otherwise gain data on 400 Al-Qaeda terrorists, making this study very important.
Understanding Terror Networks E-notes