Hi, I'm just now seeing your question, but hopefully I can provide some help.

There's a few books that I see as essential to anyone studying terrorism (beyond the already mentioned book by Chaliand and Blin) The books below are primarily overviews of terrorism, so if you'd like recommendations on something more specific let me know. The list is also not in any particular order.

Walter Laqueuer, A History of Terrorism

Walter Laqueuer, The New Terrorism: Fanaticism and the Arms of Mass Destruction

Thomas Mockaitis, The "New" Terrorism: Myths and Reality. This is a very good critique of those, like Laqueur, who support the idea that contemporary terrorism is much different than previous forms of terrorism.

Mark Juergensmeyer, Terror in the Mind of God: The Global Rise of Religious Violence

Matthew Carr, The Infernal Machine: A History of TErrorism from the Assassination of Tsar Alexander II to Al-Qaeda. A very, very good narrative history of terrorism.

Alessandro Orsini, Anatomy of the Red Brigades: The Religious Mind-Set of Modern Terrorists

Deepak Gupta, Understanding Terrorism and Political Violence: The Life Cycle of Birth, Growth, Transformation, and Demise. A good book on terrorism from the perspective of a political scientist.

Claudia Verhoeven, The Odd Man Karakozov: Imperial Russia, Modernity, and the Birth of Terrorism. Theoretically dense, but very rewarding. If your just wanting an introduction to terrorism, I wouldn't read this book first.

Walter Reich, ed., Origins of Terrorism: Psychologies, Ideologies, Theologies, States of Mind. This book is a collection of essays by various authors. It gives a good overview of the main ways that academics look at terrorism through psychological analysis.

Lisa Stampnitzsky, Disciplining Terror: How Experts Invented "Terrorism". A great book that discusses the various ways in which American academics and government officials have conceptualized terrorism since the 1960s.

Martin A. Miller, The Foundations of Modern Terrorism: State, Society and the Dynamics of Political Violence. My personal favorite general history of terrorism. The writing isn't the best of the books on this list--although it's by no means poor--but this book is, I think, the most theoretically sophisticated.

Donatella della Porta, Clandestine Political Violence. Terrorism from a sociologist's point of view. Places terrorism within the broad context of contentious politics and political violence in general.

Joseba Zulaika, Terrorism: The Self-Fulfilling Prophecy

Bruce Hoffman, Inside Terrorism

I found The History of Terrorism, eds. Gerard Chaliand and Arnaud Blin, to be a really good read and probably a really good place to start.