First, there is the "Great Jihad Encyclopedia," which is neither an encyclopedia, nor does it have much to do with terrorism. What makes this 70 page document dangerous is that is instructs its readers on how to run a secret organization, and not get caught by the police or counter-terrorism agencies. Not paying attention to the "Great Jihad Encyclopedia," or similar sources of security wisdom, is the single most common error made by Islamic terrorists. While the advice given by the Encyclopedia is pretty basic, it has the advantage of being written in Arabic, and in a style that most young jihadis (terrorists) can understand. Whether they will pay attention is another matter.
To deal with the likelihood of getting caught, there is another important document. Actually there are several. One that is clearly labeled is, "How to Confront and Cope with Intelligence Agency Interrogators" Again, written in Arabic, in a style some semi-literate fellow (the type most likely to talk if caught) can understand. These documents tell the prospective prisoner what to expect, and how best to respond. Most of this is variations on "keep cool and keep your mouth shut." But some more positive advise is given. For example, prisoners are told to, at some point, tell their interrogator that, eventually the interrogator will be out of the interrogation business. And at that point, the jihadis will come looking for him. The jihadis are also reminded that the effete Western infidels tend to eventually let jihadis out of prison, or at least to have contact with lawyers and such. Thus the jihadi threat of retaliation has some basis in fact. Pro-jihadi groups in the West can be depended on to eventually get the names of interrogators, and put them out on the Internet. So the threat of eventual retribution is not hollow, and pointing it out if meant to give encouragement to jihadis fearful of eventual capture.
Finally, we have one of the scariest terrorist strategy manuals out there; "The Management of Savagery." This document, which is written is slightly grander Arabic, is meant for jihadi leaders. It basically points out what has worked in places like Afghanistan, Iran, Sudan and Algeria (places where Islamic radicals have taken over, or come close to doing so.) The basic strategy is to find Islamic countries with weak governments, go in there and kill lots of people, make lots of terror, and weaken the government as much as possible. Then, when there is chaos all about, the jihadis move in, restore order and take over. How's that for truth in packaging. "The Management of Savagery" is also meant to make it clear to jihadis why they have to be so savage, and to reassure them that God approves.
Many technical documents, such as instructions on how to make explosives and build bombs, also exist in Arabic. Fortunately, some of the translations are faulty, which results in deadly accidents when some jihadis are building their bombs. Many jihadis believe that the CIA is going around and posting purposely incorrect versions of these documents. Actually, that's an old trick in the espionage and counter-terrorism business, and the CIA probably does do it. But it's also true that even without bad translations, English language "how to" works, like the "Anarchists Cookbook," contain potentially disastrous errors.
While all these documents are already out there, counter-terrorism organization have found, via interrogations and monitoring the Internet, that making these documents less available, leads to fewer terrorists. Just like eliminating the al Qaeda training facilities in Afghanistan did in late 2001.
Counter-terrorist efforts on the Internet have, in the past year, shifted to the offensive. This is especially true for sites that provide access to terrorist training materials. It's not true that once something is "out there on the net", efforts to suppress it are futile. If you knock down lots of sites hosting certain documents, a lot of people who might want those materials, will not be able to obtain them. There are dozens of documents that apprentice terrorists want to get their hands on, but three of them are particularly useful for budding killers.