One interesting book on this topic is Peter R. Neumann, Michael L.R. Smith, The Strategy of Terrorism. How it works, and why it fails, Routledge, New York 2008. I have already written about it on this blog some months ago, here the post.
Then I read an older book, L. Freedman, Terrorism and International Order, Routledge, London 1986, that has a very interesting chapter written by Lawrence Freedman and titled Terrorism and Strategy that highlights important features of terrorism. First of all, the author aims at considering terrorism as a problem in military strategy, but terrorism differs from other military strategies not because it exploits violence (or its threat) in pursuit of political objectives, but “in playing on the psychology of violence. It works, not through brute force, but through the fear aroused in potential victims”. On a similar point, it is interesting another old, but insightful, analysis: Thomas Perry Thornton, Terror as a Weapon of Political Agitation (in Harry Eckstein, Internal War. Problems and Approaches, The Free Press, New York 1964) in which the author stresses what he terms the “symbolic” character of terrorist acts: “Thus, in an internal war situation, terror is a symbolic act designed to influence political behaviour by extra-normal means, entailing the use or threat of violence” (73).
Secondly, if terrorism is considered a strategy, then it “must be able to generate a particular response” using violence, however “actual violence is not necessary part of the strategy”.
Thirdly, and most important, Freedman draws a distinction that I found crucial in order to understand terrorism, and mainly modern jihadist terrorism that I consider more a kind of insurgency than a terrorist campaign. Freedman distinguishes between strategic terrorism and tactical terrorism. The former relies on terror to achieve its goal, so it thinks that this method can be decisive in itself. The latter is more frequent throughout history and employs terrorism as one of several tools.
This way to consider terrorism is the most important because it enables to link terrorism to insurgency and guerrilla warfare, and shows how the continuum of violence could evolve from low intensity violence to more open warfare. Moreover, using this dichotomy it is also possible to better understand how groups could be labeled insurgent militia in a theater of operation and terrorist group in another one. The most notorious case is the Islamic State that has been labeled terrorist group, but it was actually an insurgent movement able to conquer, even using terrorism as a tactic, vast swathes of Iraq and Syria.
In summary, in order to better understand modern terrorism and terrorism as a political phenomenon it is very useful to distinguish whether the group is just a terrorist group, that is a group that uses the strategy of terrorism and consequently is not able to control territory and has to be confronted using classic counter-terrorism tools; or the group is something else and uses terrorism just as one of its tactics when and where it could achieve its goals. In this case a complex appraoch must be adopted.