Published in 2004, Samuel Katz's book is a clear, simple, brief description of what Jihad is, where it comes from, and how it evolved to be what it is today. The book clarifies the situation in a way larger books really cannot. The brevity itself helps make things clearer.
This is not to say that larger books aren't also helpful to read, but to begin your exploration of this important topic, or to help clarify and put in perspective the information you've read so far, Jihad: Islamic Fundamentalist Terrorism is my first choice. It will give you the big picture.
I have often enjoyed books written for young people, and this book has a young audience in mind. Books aimed at teens tend to avoid getting lost in detail and present the important and interesting points in starker relief.
Jihad starts out with the history of Islam itself, and then the history of the fundamentalist movement — where it began and why. Then there are specific histories of the Jihad warriors of Egypt, the Hezbollah of Lebanon, the major terrorist group in Algeria, and finally Al-Qaeda's international network.
Samuel M. Katz is an expert in the field of international terrorism and counterterrorism, and serves as the editor in chief of Special Ops.