In 1999 the RAND Corporation published a study they made of the "new" terrorism for the United States Air Force, under the title, Countering the New Terrorism by Ian O. Lesser, et al. (ISBN: 0833026674). In it, the authors discuss the new terrorist movements (including Al-Qaeda among others), their non-hierarchical organization and their propensity for extremely lethal violence. The final chapter covers how to counter these terrorist movements.
Although the book is not about Al-Qaeda in particular, I found that it did go a long way towards explaining how the organization works, why no one is claiming credit for the recent attack, and why removing bin Laden from the scene will not stop Al-Qaeda's attacks. Also, I have found that this book goes a long way towards clarifying what President Bush is saying and apparently planning. If the authors of this study are correct, then the President is quite correct in assuring the American public that this war will have to be waged over an extended period.
I was lucky enough to be given a copy of this book by a colleague, but if you can obtain a copy I do think that it would be worth you time to read it.
On page 88, the authors wrote, "Until recent years, however, few of these [terrorist] attacks took place within the United States, partly because traditional terrorist groups found the prospect of operations in the United States too difficult, politically counterproductive, or simply unnecessary. Most observers now believe the threshold for significant international terrorism in the United States has been crossed..."