A highly important source is The Cyber Vault Project, which is being built and maintained by researchers at George Washington University as part of a project known as the National Security Archive.
The researchers submit requests for information through the Freedom of Information Act (FOIA) and gain access to documents that were considered classified.
Thus, we gained access to fascinating documents on issues such as the US Cyber Command operation against ISIS, the US Electric Infrastructure Protection Assessments, and more.
From these documents, we can learn a lot about the considerations that led decision-makers to execute cyber operations and how they coped with their results.
Highly recommended to anyone interested or researching cyber policy and strategy.