Academic literature on the construction of dangerous things
For the US Department of Defense there is the Defense Technical Information Center (DTIC). It is like PubMed Central but for DoD documents. Some of the content has been cleared for public release and is open to the public. Much of the content, however, has not been cleared for public release and access is controlled by a DoD employees common access card (CAC) and the network that DTIC is being accessed from. Not cleared for public release is not the same as classified/secret/top secret. Access to classified material is again CAC controlled and requires a dedicated computer on a protected network. I think, but am not sure, that DTIC is the gateway to much of the classified literature within the classified network.