I know that I'm pretty behind the times but I just read this book and learned a lot so I thought I would recommend it.
The most pertinent question I had going into the book was what attracted people to radical Islam and the terrorist activity that Al Qaeda employed. Obviously on the Huffington Post we see endless blame directed at Israel, but that's only part of the story, especially since al Qaeda doesn't often attack Israel. There were two kinds of people who joined up with Osama and his friends: Millionaires like Osama himself and well off Muslims who often lived in the West before going to Yemen or Afghanistan for training in "martyrdom" activities. Very few people came who were dirt poor, partially because the terror groups needed money more than they needed people.
So why did these Muslims get indoctrinated into these radical beliefs? Osama himself was very religious and came to Qutb's writings on his own. His main motivation was against his own Saudi government for not only being too secular (really) but also allying with us Americans. As for the foot soldiers there were two reasons: the first were Arabs who came from Arab countries but whose lives had no direction. They weren't going to school, they didn't have much chance of employment, they weren't going to get married in all likelihood. They didn't have much to live for so why not find a cause to die for? Now maybe you can blame the USA for the economic troubles of the Arab world but that's a bit more of a stretch. The second group of "martyrs" were well educated Muslims who lived in the West, as I said. The problem was that they were looking for an identity as they felt like they weren't "real" Muslims because they weren't from the Middle East, but they also weren't "real" Westerners because being Muslim still mattered a lot to them and was part of their identity. This made them susceptible to the sway of radical Islam.
The first half of the book was about Osama's "mujaheddin" activities in Afghanistan, which was very informative as well. As you know a lot of left-wingers believe that Osama only became powerful because "we" (as in America) financed him against the Soviets in Afghanistan. But that isn't the case. Osama was not financed by America, the Afghani fighters were. Osama and his Arab friends showed up in Afghanistan intending to die for the cause, but because they were so incompetent and fanatical the Afghanis kept them off the front lines and away from most of the fighting. A clear example of this is when everyone was at the camp site and Soviet warplanes flew overhead, the Arabs actually ran out of the foxholes and yelled "please kill us." The goal was not to win, but to die gloriously.
I don't want to ramble, as there's a lot more to discuss in the book, like how an FBI agent named John O'Neill was one of the leaders in tracking Osama only to be ignored and ultimately to die in the attacks. There's a lot of "Legacy Of Ashes," about how the CIA and FBI dropped the ball, as well as a history of Islamic terror such as the USS Cole bombing and the first attack on the WTC. It's not very long and very well written, so if this is something you're interested in I suggest you give it a try.