A Military Guide to Terrorism in the Twenty-First Century
U.S. Army Training and Doctrine Command 182 page Acrobat file
One of the things the military does well is research. A Military Guide to Terrorism in the Twenty-First Century is certainly an example of this, and will be of interest to anyone who deals with terrorism or is interested in terrorism. It is a comprehensive, inclusive overview.
While by definition aimed at the military, this does not, in fact, limit its utility. As an example, those who deal with protection will find that Appendix A, which deals with the terrorist planning cycle, will serve as a good refresher, and a reminder of the importance of regular surveillance operations against their own facilities or principals.
If you have an interest in knowing about terrorism, this free download is a great resource with which to start.